doc(auth): add PQC product neutral guide

[scotthart]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces a new product-neutral guide on Post-Quantum Cryptography (PQC) and integrates it into the CMake build configuration. The feedback recommends improving the documentation by updating the code block language identifier from 'c' to 'cpp' for correct syntax highlighting, and replacing the use of 'auto' with an explicit type in the C++ example to align with the repository's style guide regarding obscured return types.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.39%. Comparing base (7dc9b68) to head (f1243ee).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16548      +/-   ##
==========================================
- Coverage   92.41%   92.39%   -0.03%     
==========================================
  Files        2262     2262              
  Lines      218382   218382              
==========================================
- Hits       201819   201771      -48     
- Misses      16563    16611      +48     

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