add review comment to sb files

[oleksandr-didyk]

Contributes to dotnet/source-build#3435

Adds comments to source-build files asking for the inclusion of the source-build team in PRs that alter SourceBuild* files. Non-reviewed changes could potentially cause issues down the line, be it in the downstream repos or the product build (as has happened in the past, see dotnet/source-build#3435 (comment))

[codecov]

Codecov Report

Merging #6666 (cfa623a) into main (f4cc17e) will decrease coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6666      +/-   ##
==========================================
- Coverage   96.36%   96.36%   -0.01%     
==========================================
  Files        1390     1390              
  Lines      324039   324039              
  Branches    10613    10613              
==========================================
- Hits       312274   312271       -3     
- Misses       9224     9226       +2     
- Partials     2541     2542       +1     

[oleksandr-didyk]

@dotnet/roslyn-analysis soft ping - would be great if someone could take a look at this small PR. Thank you!

[oleksandr-didyk]

Reached out to @sharwell in DMs - agreed to add a CODEOWNERS entry for affected source-build files and write permissions for the @dotnet/source-build-internal team alias to make the entry valid