docs(front-matter): improve docs for stabilization

[magurotuna]

This commit improves docs of the front-matter module.

Specifically, the following things are done:

• Use @example where appropriate to render the examples more beautifully
• Split the big example attached to createExtractor function into smaller pieces so that each one has minimal yet meaningful example
• Add @returns to the test function
• Deduplicate Format type definition
• Add a brief description to Format type

Towards #3764

[CLAassistant]

All committers have signed the CLA.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.35%. Comparing base (083b63f) to head (d919f94).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4789      +/-   ##
==========================================
- Coverage   92.36%   92.35%   -0.01%     
==========================================
  Files         487      487              
  Lines       41539    41539              
  Branches     5405     5405              
==========================================
- Hits        38366    38364       -2     
- Misses       3117     3119       +2     
  Partials       56       56              

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