Add some documentation about registries.conf

[jonesbusy]

Description

Add some documentation about registries.conf

Testing done

None

Submitter checklist

• I have read and understood the CONTRIBUTING guide
• I have run mvn license:update-file-header, mvn spotless:apply, pre-commit run -a, mvn clean install before opening the PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.62%. Comparing base (4abf734) to head (665b1cb).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #564      +/-   ##
============================================
+ Coverage     87.91%   88.62%   +0.71%     
  Complexity      800      800              
============================================
  Files            42       42              
  Lines          2374     2374              
  Branches        292      292              
============================================
+ Hits           2087     2104      +17     
+ Misses          175      158      -17     
  Partials        112      112              

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