Docs: move documentation to README

[ohmyfelix]

Summary

• move .docs/README.md content into the root README.md
• keep the existing Contributte README header, badges, version table, development section, and footer
• remove the old root README pointer to [documentation](.docs)
• delete the moved .docs/README.md

Refs contributte/contributte#75

Verification

• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.07%. Comparing base (0d0e339) to head (cf450fd).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #90   +/-   ##
=======================================
  Coverage   96.07%   96.07%           
=======================================
  Files           5        5           
  Lines         102      102           
=======================================
  Hits           98       98           
  Misses          4        4           

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