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

scripts/Dockerfile-ubuntu_21_04

@@ -1,5 +1,8 @@
 FROM ubuntu:21.04 AS sunshine-ubuntu_21_04
 
+# Ubuntu 21.04 end of life is January 2022
+# This file remains for reference only
+
 ARG DEBIAN_FRONTEND=noninteractive 
 ARG TZ="Europe/London"