Temporarily disable PDF build

[hakonanes]

Description of the change

This PR hopefully fixes the currently failing RTD PDF build by converting SVGs to supported formats. https://github.com/sphinx-doc/sphinx/issues/1907 recommended using sphinx.ext.imgconverter, which converts automatically using imagemagick, so this is installed on the RTD server prior to building the docs.

The above is fixed, but PDF build errors, so we disable it for now.

Progress of the PR

• [n/a] Docstrings for all functions
• [n/a] Unit tests with pytest for all lines
• [n/a] Clean code style by running black via pre-commit

For reviewers

• [ ] The PR title is short, concise, and will make sense 1 year later.
• [ ] New functions are imported in corresponding __init__.py.
• [ ] New features, API changes, and deprecations are mentioned in the unreleased section in CHANGELOG.rst.
• [ ] New contributors are added to release.py, .zenodo.json and .all-contributorsrc with the table regenerated.

[hakonanes]

Decided to disable PDF build for now as the log displays no error except a message from latexmk stating there was some error (https://readthedocs.org/projects/kikuchipy-hakonanes/builds/19785794/). Closest related issue is https://github.com/readthedocs/readthedocs.org/issues/10150.