Docs: Use doxygen-awesome-css theme (#6512)

* add doxygen-theme-css submodule

* enable theme and disable not needed code references

* hide nav sync button

* css and cleanup

* Move comments to dedicated README to not fail config check

doc/doxygen/extra-pages/index.md

@@ -1,10 +1,8 @@
-@mainpage Cockatrice Documentation
+@mainpage Documentation
 
-# Welcome
+Welcome to the Cockatrice code documentation.
 
-This is the **main landing page** of the Cockatrice documentation.
+- @subpage user_reference
+- @subpage developer_reference
 
-- Go to the @subpage user_reference page
-- Review the @subpage developer_reference
-
-Or check out the <a href="https://cockatrice.github.io/" target="_blank" rel="noopener noreferrer">Cockatrice Webpage</a>.
+Please also check the <a href="https://cockatrice.github.io/" target="_blank" rel="noopener noreferrer">Cockatrice Webpage</a> or our <a href="https://github.com/Cockatrice/Cockatrice" target="_blank" rel="noopener noreferrer">Code Repository</a> for general information.

doc/doxygen/extra-pages/user_documentation/index.md

@@ -1,17 +1,17 @@
 @page user_reference User Reference
  
-# Deck Management
+## Deck Management
 
 - @subpage creating_decks
 - @subpage importing_decks
 - @subpage editing_decks
 - @subpage exporting_decks
 
-# Release Channels
+## Release Channels
 
 - @subpage beta_release
 
-# Syntax Help
+## Syntax Help
 
 - @subpage search_syntax_help
-- @subpage deck_search_syntax_help
+- @subpage deck_search_syntax_help