Refactor to only require pymatgen-core, and fix docs building

[kavanase]

This refactors the imports and dependencies of pymatgen-analysis-defects so that it only requires pymatgen-core, and not the full extended pymatgen suite. All tests pass locally.

I also noticed that the pmg analysis defects docs: https://materialsproject.github.io/pymatgen-analysis-defects/ is not rendering. This PR includes a fix which may resolve that (though I'm not sure if it will fully resolve that issue).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.94%. Comparing base (12c1f80) to head (1fcb699).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #224   +/-   ##
=======================================
  Coverage   90.94%   90.94%           
=======================================
  Files          14       14           
  Lines        2265     2265           
  Branches      287      287           
=======================================
  Hits         2060     2060           
  Misses        118      118           
  Partials       87       87           

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[jmmshn]

Heads up — this PR fully subsumes the fix for #225 (docs not building):

• Drops the broken develop-branch gate on the docs Build step in testing.yml.
• Pins jupyter-book>=0.13.1,<2 in pyproject.toml (jupyter-book 2.x removed the --path-output flag the build invocation depends on).

Both changes are exactly what's needed to bring docs back online. Once this merges I'll close #225. Thanks for the contribution!

[kavanase]

Just bumping this! Not sure if there was a reason not to merge before