Add examples to fit function docstrings

[Astro-Kirsty]

This is to add examples to various fit functions, as mention in comment here https://github.com/gammapy/gammapy/issues/5291#issuecomment-2163318629

[codecov[bot]]

Codecov Report

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

Project coverage is 94.48%. Comparing base (64cdfa2) to head (97cdb5e). Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5346   +/-   ##
=======================================
  Coverage   94.48%   94.48%           
=======================================
  Files         232      232           
  Lines       35742    35742           
=======================================
  Hits        33772    33772           
  Misses       1970     1970           

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