- Improve formatting of `AUTHORS.md`, `CONTRIBUTING.md`, `LICENSE.md`, `README.md` and `SECURITY.md`
- Update wording to include `Mailu-OIDC`-related information in `AUTHORS.md`, `CONTRIBUTING.md`, `LICENSE.md`, `README.md`, `RELEASE_TEMPLATE.md`, `SECURITY.md`
- Add OIDC setup guide to `README.md`
- Switch from issue template to issue forms
- Add pull-request templates