[DOC] Document nlohmann-json dependency maintenance

[yigitcan-ozturk]

Changes

• add nlohmann-json to the dependency-maintenance overview
• document its upstream repository and release source
• document the version/update locations used by the submodule, CMake, Bazel, and Conan paths
• add validation notes so future upgrades keep package-manager declarations aligned

This is documentation-only and does not change build or runtime behavior.

Refs #2678

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.42%. Comparing base (bcd4f2d) to head (4f4d226).
⚠️ Report is 24 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4561      +/-   ##
==========================================
+ Coverage   83.47%   86.42%   +2.95%     
==========================================
  Files         521      524       +3     
  Lines       20380    20435      +55     
==========================================
+ Hits        17011    17659     +648     
+ Misses       3369     2776     -593     

see 63 files with indirect coverage changes

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