docs: add CMake getting-started guide for new users

[leno23]

Summary

Adds a Getting started with CMake section that answers common first-time questions from #2351:

• Where CMake snippets belong (CMakeLists.txt in the project root)
• Minimal directory layout (CMakeLists.txt + test.cpp)
• When to use Catch2::Catch2WithMain vs Catch2::Catch2 (custom main)
• A smallest working CMakeLists.txt and build/run commands

Also links to this section from tutorial.md.

Fixes #2351.

Test plan

• Documentation-only change; markdown renders correctly.

Made with Cursor

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.31%. Comparing base (69e0473) to head (21e20c5).

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #3105      +/-   ##
==========================================
+ Coverage   91.26%   91.31%   +0.04%     
==========================================
  Files         204      204              
  Lines        8905     8905              
==========================================
+ Hits         8127     8131       +4     
+ Misses        778      774       -4     

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