Improve Optimization Detective documentation

[westonruter]

This is a follow-up to #1763.

• Remove @access private from interfaces which are intended to be exposed.
• How to write an extension
• Class reference
• Function reference
• JSON Schema
• Remove access private from anything exposed by filters or hooks
• Extract hooks docs from PHPDoc? This could should include improved formatting of the accepted arguments to each action/filter.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.38%. Comparing base (dde008b) to head (32a95ab).
Report is 14 commits behind head on trunk.

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #1782   +/-   ##
=======================================
  Coverage   57.38%   57.38%           
=======================================
  Files          84       84           
  Lines        6504     6504           
=======================================
  Hits         3732     3732           
  Misses       2772     2772           

Flag	Coverage Δ
multisite	57.38% <ø> (ø)
single	34.42% <ø> (ø)

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