[editorial] Add headings to configauth/README.md

[chalin]

• Adds headings so as to have anchor IDs that the docs can refer to.
• This is proposed, in part, to help address Collector/building/... : links to specific authenticators are invalid opentelemetry.io#6234

/cc @open-telemetry/docs-maintainers

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.41%. Comparing base (43087bd) to head (d8e8209).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12309   +/-   ##
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  Coverage   91.41%   91.41%           
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  Files         467      467           
  Lines       25583    25583           
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  Hits        23387    23387           
  Misses       1778     1778           
  Partials      418      418           

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