Docs: move documentation to README

[ohmyfelix]

What changed

• Moved the documentation index from .docs/README.md into the root README.md.
• Reworked the root README order to match the Contributte README style with versions and installation early.
• Updated documentation links to point at .docs/*.md from the root README.

Why

Refs contributte/contributte#75.

Testing

• git diff --check
• Targeted README scans for duplicate headings, stale .docs/README links, and moved .docs/*.md links.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.77%. Comparing base (d9144ab) to head (ff824ac).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1292   +/-   ##
=======================================
  Coverage   48.77%   48.77%           
=======================================
  Files          61       61           
  Lines        2901     2901           
=======================================
  Hits         1415     1415           
  Misses       1486     1486           

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