Update docs v3

[isabella200617]

Description

This PR updates outdated documentation references for Catch2 v3 to ensure the documentation accurately reflects the current state of the project.

Changes made:

• contributing.md: Clarified that v3 is the main development branch, removing outdated references to v2.x active maintenance
• own-main.md: Fixed broken Clara documentation link to point to internal command-line documentation instead of archived external repository
• migrate-v2-to-v3.md: Improved migration instructions to be less destructive (removing definitions vs deleting files)
• release-notes.md: Updated outdated include references from catch.hpp to catch_all.hpp for v3 compatibility

These changes help users:

• Understand the current development focus is on v3
• Follow working documentation links
• Successfully migrate from v2 to v3 without confusion
• Use correct include paths in their projects

GitHub Issues

Addresses #2050

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.04%. Comparing base (a1faad9) to head (1068823).

Additional details and impacted files

@@           Coverage Diff           @@
##            devel    #3046   +/-   ##
=======================================
  Coverage   91.04%   91.04%           
=======================================
  Files         202      202           
  Lines        8716     8716           
=======================================
  Hits         7935     7935           
  Misses        781      781           

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