docs: document embedded Clara and drop archived upstream link (#2312)

[annacastroops]

Summary

Adds Catch2-owned Clara documentation and stops pointing users at the archived standalone Clara repository (#2312).

Clara has lived only inside Catch2 for years (src/catch2/internal/catch_clara.hpp). The old upstream README is inactive and no longer matches the embedded parser.

Changes

• New docs/clara.md covering Opt / Arg / Help composition used with session.cli()
• docs/own-main.md: link to the new page
• docs/Readme.md: index entry under Fine tuning

Closes #2312

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.27%. Comparing base (0aeb818) to head (c44cfe6).

Additional details and impacted files

@@           Coverage Diff           @@
##            devel    #3191   +/-   ##
=======================================
  Coverage   91.27%   91.27%           
=======================================
  Files         204      204           
  Lines        8973     8973           
=======================================
  Hits         8190     8190           
  Misses        783      783           

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