docs: use the default theme on ReadTheDocs.org as well

Historically, we've been using the RTD theme on the RTD site which hosts
the docs for us, and a Sphinx-default, "Alabaster" theme for other docs
builds. Doing that however started failing:

 Traceback (most recent call last):
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/builders/html/__init__.py", line 1096, in handle_page
     output = self.templates.render(templatename, ctx)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/readthedocs_ext/readthedocs.py", line 181, in rtd_render
     content = old_render(template, render_context)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/jinja2glue.py", line 194, in render
     return self.environment.get_template(template).render(context)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/jinja2/environment.py", line 1301, in render
     self.environment.handle_exception()
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/jinja2/environment.py", line 936, in handle_exception
     raise rewrite_traceback_stack(source=source)
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/themes/basic/page.html", line 10, in top-level template code
     {%- extends "layout.html" %}
     ^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/themes/classic/layout.html", line 10, in top-level template code
     {%- extends "basic/layout.html" %}
     ^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/themes/default/../basic/layout.html", line 170, in top-level template code
     {%- block content %}
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/themes/default/../basic/layout.html", line 189, in block 'content'
     {%- block sidebar2 %}{{ sidebar() }}{% endblock %}
     ^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/themes/default/../basic/layout.html", line 189, in block 'sidebar2'
     {%- block sidebar2 %}{{ sidebar() }}{% endblock %}
     ^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/jinja2/sandbox.py", line 393, in call
     return __context.call(__obj, *args, **kwargs)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/jinja2/runtime.py", line 777, in _invoke
     rv = self._func(*arguments)
          ^^^^^^^^^^^^^^^^^^^^^^
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/themes/default/../basic/layout.html", line 63, in template
     {%- include sidebartemplate %}
     ^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/jinja2glue.py", line 215, in get_source
     raise TemplateNotFound(template)
 jinja2.exceptions.TemplateNotFound: about.html

 The above exception was the direct cause of the following exception:

 Traceback (most recent call last):
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/cmd/build.py", line 281, in build_main
     app.build(args.force_all, args.filenames)
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/application.py", line 347, in build
     self.builder.build_update()
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/builders/__init__.py", line 310, in build_update
     self.build(to_build,
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/builders/__init__.py", line 376, in build
     self.write(docnames, list(updated_docnames), method)
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/builders/__init__.py", line 571, in write
     self._write_serial(sorted(docnames))
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/builders/__init__.py", line 581, in _write_serial
     self.write_doc(docname, doctree)
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/builders/html/__init__.py", line 672, in write_doc
     self.handle_page(docname, ctx, event_arg=doctree)
   File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/builders/html/__init__.py", line 1103, in handle_page
     raise ThemeError(__("An error happened in rendering the page %s.\nReason: %r") %
 sphinx.errors.ThemeError: An error happened in rendering the page about-project.
 Reason: TemplateNotFound('about.html')

 Theme error:
 An error happened in rendering the page about-project.
 Reason: TemplateNotFound('about.html')

I have no clue what that means because we have never requested this
`about.html`, nor do we reference that file from anywhere. Chances are
that it's "just" some version pinning/compatibility issue, but hey --
why mess with that when there's a perfectly good default theme that
we're using for other purposes already.

As a side effect, this also solves that long-standing issue that Esther
reported where the tables have overly long lines. Apparently, it's a
theme-specific misfeature (readthedocs/sphinx_rtd_theme/#117), and the
Alabaster one doesn't suffer from that.

All hail alabaster!

Change-Id: I857890f29f14b7c0f66bca201c9a9c1b1cbf8841

docs/conf.py

@@ -84,13 +84,6 @@ todo_include_todos = False
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
 #
-on_rtd = os.environ.get('READTHEDOCS') == 'True'
-if on_rtd:
-    html_theme = 'default'
-    html_theme_options = {
-        'logo_only': True,
-    }
-else:
 html_theme = 'alabaster'
 html_theme_options = {
     'logo': 'images/GNPy-logo.png',