Updates the k8s helm platform docs (#8632)

* Updates the k8s helm platform docs

- Updates to talk about the external mode
- Updates the helm install overview to show that the releases can also
  be the way to install
- Rewrites the how-to to include showing how to start in each mode
- Each mode that has a guide links off to a guide
- Re-organizes the Unseal and Init to a section and places all the
  various other unseals underneath it
- Moves updating below the unseal and init
- Shows some basic usage of the helm CLI with a value and file override

* Adds learn links for k8s index pages

* Adds helm dev and external vault examples

While the dev one may seem obvious I think that it's incredibly useful
to cover our bases if this is to be reference documentation. I thought
maybe the example could have ingress support for UI but do not have the
experience to recommend it.

* Adds helm docs example dev and external

- places the development first as it feels like the starting point for
  some.
- places the external after HA

Co-authored-by: Vishal Nayak <vishalnayak@users.noreply.github.com>

website/data/docs-navigation.js

@@ -319,10 +319,12 @@ export default [
               {
                 category: 'examples',
                 content: [
+                  'development',
                   'standalone-load-balanced-ui',
                   'standalone-tls',
                   'standalone-audit',
                   'ha-with-consul',
+                  'external',
                   'kubernetes-auth',
                   'enterprise-best-practice'
                 ]