docs: add Docusaurus site, deploy workflows, and content refresh

[leehack]

Summary

• add a Docusaurus docs site under website/ with reorganized llamadart documentation
• add docs automation workflows (docs_pages.yml, docs_version_cut.yml)
• add docs build/link scripts under tool/docs
• improve content accuracy (platform/backend matrix, migration notes, web bridge notes)
• improve readability of key diagrams and landing/docs theming

Validation

• ./tool/docs/validate_links.sh

Notes

• branch: docs_site_refresh
• after merge to main, docs_pages.yml will deploy the docs site

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.37%. Comparing base (46b5f11) to head (114e934).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #56   +/-   ##
=======================================
  Coverage   76.37%   76.37%           
=======================================
  Files          65       65           
  Lines        7915     7915           
=======================================
  Hits         6045     6045           
  Misses       1870     1870           

Flag	Coverage Δ
unittests	76.37% <ø> (ø)

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