Docs: move documentation to README

[ohmyfelix]

What changed

• Moved .docs/README.md content into the root README.md.
• Removed the .docs/README.md pointer target after migration.
• Kept the Contributte badge/contact scaffold and placed the version matrix before installation.
• Fixed README table-of-contents anchors and small PHP snippet typos while moving the docs.

Why

Refs contributte/contributte#75.

Testing

• git diff --check
• Targeted README scans for duplicate heading anchors, stale .docs references, and broken local TOC anchors.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.00%. Comparing base (757e8ea) to head (2f685b3).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #55   +/-   ##
=======================================
  Coverage   90.00%   90.00%           
=======================================
  Files          15       15           
  Lines         160      160           
=======================================
  Hits          144      144           
  Misses         16       16           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.