Expand documentation build troubleshooting guide to discuss missing toctrees

[namurphy]

This PR expands the troubleshooting section of the documentation guide to include a description of what to do when we encounter documentation build warnings related to missing toctrees.

This warning arose in #2191.

[github-actions]

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Our contributor guide has information on:

• Getting ready to contribute and the code contribution workflow
• Using astropy.units and plasmapy.particles
• Coding tips and guidelines
• Writing documentation (in reStructuredText with numpydoc style docstrings).
• Writing tests using pytest
• Adding a changelog entry (needed except for minor changes)

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[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (99f905b) 98.35% compared to head (c9cd132) 98.35%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2257   +/-   ##
=======================================
  Coverage   98.35%   98.35%           
=======================================
  Files         101      101           
  Lines        8580     8580           
=======================================
  Hits         8439     8439           
  Misses        141      141           

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[ejohnson-96]

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