docs: in-repo Clara documentation (fixes #2312)

[wyf027]

Summary

Fixes #2312.

Adds docs/clara.md describing the embedded Catch::Clara API and points own-main.md / opensource-users.md at it instead of the archived standalone Clara repository.

Test plan

• Markdown links only

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.26%. Comparing base (69e0473) to head (3ca196b).

Additional details and impacted files

@@           Coverage Diff           @@
##            devel    #3108   +/-   ##
=======================================
  Coverage   91.26%   91.26%           
=======================================
  Files         204      204           
  Lines        8905     8905           
=======================================
  Hits         8127     8127           
  Misses        778      778           

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[wyf027]

Friendly bump: this PR adds a short in-repo docs/clara.md hub and updates the two stale external Clara links (own-main.md, opensource-users.md) for #2312.

Compared to #3062, this keeps the archived-repo context and links to 232-Cfg-CustomMain.cpp + command-line.md. Happy to rebase or consolidate if maintainers prefer a single approach.

[wyf027]

Friendly bump on #2312 — still open and the external Clara links in own-main.md / opensource-users.md remain stale. This PR is ready for review when you have time.

[wyf027]

Friendly bump — docs-only PR linking Clara usage to in-repo clara.md (Fixes #2312). Ready for review when you have time.