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/index.rst

@@ -0,0 +1,7 @@
+:github_url: https://github.com/SunshineStream/Sunshine/tree/nightly/docs/source/index.rst
+
+SunshineStream has this documentation hosted on `Read the Docs <http://sunshinestream.readthedocs.io/>`_.
+
+Table of Contents
+=================
+.. include:: toc.rst