docs: expand documentation and improve docs website UX

[mckaragoz]

Summary

Expands and refines UltimateAuth documentation while improving the documentation website's navigation and usability.

Changes

• Expanded Getting Started and Authentication Flow documentation.
• Improved documentation sidebar, table of contents, and page navigation.
• Refined responsive layout, styling, and theme preference persistence.
• Updated generated documentation content and indexing.

Notes

Theme preferences persist across reloads. A brief theme transition may occur during WebAssembly initialization due to prerendering.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.98%. Comparing base (b5044c8) to head (8bf50ab).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev      #78   +/-   ##
=======================================
  Coverage   85.98%   85.98%           
=======================================
  Files         660      660           
  Lines       18275    18275           
  Branches     2249     2249           
=======================================
  Hits        15714    15714           
  Misses       1810     1810           
  Partials      751      751           

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