Initial version of sphinx documentation and...

- remove ubuntu 21.04 from CI (end of life)
- adjust matrix strategy for clang.yml
- Use lessons learned from RetroArcher on localize.yml, crowdin.yml, and locale.py
- Add end of life comments to Dockerfiles
- Adjust dependency order in Dockerfiles

docs/source/contributing/contributing.rst

@@ -0,0 +1,32 @@
+:github_url: https://github.com/SunshineStream/Sunshine/tree/nightly/docs/source/contributing/contributing.rst
+
+Contributing
+============
+#. Fork the repo on GitHub
+#. Create a new branch for the feature you are adding or the issue you are fixing
+
+   .. Tip:: Base the new branch off the `nightly` branch. It will make your life easier when you submit the PR!
+
+#. Make changes, push commits, etc.
+#. Files should contain an empty line at the end.
+#. Document your code!
+#. Test your code!
+#. When ready create a PR to this repo on the `nightly` branch.
+
+   .. Hint:: If you accidentally make your PR against a different branch, a bot will comment letting you know it's on
+      the wrong branch. Don't worry. You can edit the PR to change the target branch. There is no reason to close the
+      PR!
+
+   .. Note:: Draft PRs are also welcome as you work through issues. The benefit of creating a draft PR is that an
+      automated build can run in a github runner.
+
+   .. Attention:: Do not expect partially complete PRs to be merged. These topics will be considered before merging.
+
+      - Does the code follows the style guidelines of this project?
+
+         .. Tip:: Look at examples of existing code in the project!
+
+      - Is the code well commented?
+      - Were documentation blocks updated for new or modified components?
+
+   .. Note:: Developers and maintainers will attempt to assist with challenging issues.