[repo] Fix docfx build + improve CI for md-only PRs

[CodeBlanch]

[It looks like #5246 introduced an issue into the docfx build which was able to sneak by CI.]

Changes

• Run docfx build as part of CI if markdown files are being modified.
• Include Directory.Packages.prop as content for docfx.
• Include docs\logs\README.md in the solution.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)

[CodeBlanch]

See: https://github.com/open-telemetry/opentelemetry-dotnet/actions/runs/7658805846/job/20872556269?pr=5256#step:4:832

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6250307) 83.38% compared to head (f18cec0) 83.12%.
Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5257      +/-   ##
==========================================
- Coverage   83.38%   83.12%   -0.27%     
==========================================
  Files         297      271      -26     
  Lines       12531    11970     -561     
==========================================
- Hits        10449     9950     -499     
+ Misses       2082     2020      -62     

Flag	Coverage Δ
unittests	?
unittests-Instrumentation-Experimental	25.07% <83.33%> (?)
unittests-Instrumentation-Stable	25.07% <83.33%> (?)
unittests-Solution-Experimental	83.10% <100.00%> (?)
unittests-Solution-Stable	83.12% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...tation.AspNetCore/Implementation/HttpInListener.cs	89.72% <100.00%> (+0.14%)	⬆️
...AspNetCore/Implementation/HttpInMetricsListener.cs	89.47% <ø> (ø)
...plementation/HttpWebRequestActivitySource.netfx.cs	80.77% <100.00%> (ø)
...emetry/Metrics/Exemplar/SimpleExemplarReservoir.cs	80.43% <100.00%> (ø)

... and 33 files with indirect coverage changes