[otlp] Add readme toc and clarify endpoint confusion

[vishweshbankwar]

Fixes #
Design discussion issue #

Changes

Adds toc to otlp exporter Readme as it is getting bigger. Readme needs more organization, planning to do that separately. Added a note about endpoint confusion when using HttpProtobuf #5377

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.31%. Comparing base (6250307) to head (c0a67a8).
Report is 126 commits behind head on main.

❗ Current head c0a67a8 differs from pull request most recent head 9b77cf0. Consider uploading reports for the commit 9b77cf0 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5385      +/-   ##
==========================================
- Coverage   83.38%   83.31%   -0.08%     
==========================================
  Files         297      278      -19     
  Lines       12531    11936     -595     
==========================================
- Hits        10449     9944     -505     
+ Misses       2082     1992      -90     

Flag	Coverage Δ
unittests	?
unittests-Solution-Experimental	83.04% <ø> (?)
unittests-Solution-Stable	83.27% <ø> (?)

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

Files	Coverage Δ
...orter.OpenTelemetryProtocol/OtlpExporterOptions.cs	96.36% <ø> (ø)

... and 48 files with indirect coverage changes

[github-actions]

This PR was marked stale due to lack of activity and will be closed in 7 days. Commenting or Pushing will instruct the bot to automatically remove the label. This bot runs once per day.