[DOC] - Update / extend documentation

[TomDonoghue]

Includes the addition of new documentation materials:

• new section in the examples for customizing fit approaches
• new example: custom metrics
• new example: custom fit modes

Additionally, includes the following updates to existing documentation materials:

• update AP fit tut to include doublexp

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.50%. Comparing base (9b39897) to head (d48dc31).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #370   +/-   ##
=======================================
  Coverage   97.50%   97.50%           
=======================================
  Files         150      150           
  Lines        6365     6365           
=======================================
  Hits         6206     6206           
  Misses        159      159           

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