docs: refresh project documentation

[llbbl]

Summary

• refocus the README into a concise, value-first landing page for the project
• move setup, deployment, and reference detail into dedicated docs pages
• refresh semantic search docs with stable provider-contract and 384-dimension embedding wording

Changes

Documentation

• README.md: shorten the project entrypoint and link to detailed documentation
• docs/README.md: add the docs index for deeper guides
• docs/GETTING_STARTED.md: document installation, local setup, and indexing flow
• docs/DEPLOYMENT.md: document deployment requirements and build/index steps
• docs/REFERENCE.md: centralize configuration and operational reference details
• content/features/semantic-search.md: correct embedding and provider-contract language
• content/theme/overview.md: refresh theme/content docs for the current project framing

Validation

• pnpm format
• pnpm lint
• pnpm exec tsc --noEmit
• pnpm test:coverage --run (178 passed, 96.54% lines)
• pnpm db:init:local
• pnpm index:local
• TURSO_DB_URL=file:local.db TURSO_AUTH_TOKEN=local pnpm build
• Local link target checks
• Independent docs review PASS after one finding was fixed

Closes #86
Closes #91

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.21%. Comparing base (dcd7abb) to head (6c0a519).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #94   +/-   ##
=======================================
  Coverage   91.21%   91.21%           
=======================================
  Files          12       12           
  Lines         330      330           
  Branches       86       86           
=======================================
  Hits          301      301           
  Misses         11       11           
  Partials       18       18           

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