Add sphinx.ext.imgconverter extension to make SVGs build in LateX

[clelange]

Docs are failing to build because there's no SVG support, see an example log at https://readthedocs.org/api/v2/build/22295173.txt

This is an attempt to fix the issue, following sphinx-doc/sphinx#1907

---

📚 Documentation preview 📚: https://hepdata-lib--240.org.readthedocs.build/en/240/

[GraemeWatt]

Sorry, I only checked the Sphinx build locally using make html and not the other formats. For pull requests, "Additional formats like PDF and EPUB aren’t built, to reduce build time.". I checked that I can locally run make latexpdf successfully after your fix to add sphinx.ext.imgconverter. Running make epub gives a warning message "WARNING: unknown mimetype for .nojekyll, ignoring" (also seen at the bottom of the build log). This can be suppressed by adding suppress_warnings = ['epub.unknown_project_files'] to docs/conf.py (see sphinx-doc/sphinx#3214). I guess the new "Building the documentation" section in docs/dev.rst should mention that the developer should check the Sphinx build locally with make latexpdf and make epub. Alternatively, if only HTML documentation is needed, the formats: all line could simply be removed from .readthedocs.yml.

[clelange]

Thanks for the suggestions @GraemeWatt -- I've now also suppressed the epub .nojekyll warning and added a note on testing LateX and epub docs. I think it might still be useful to build those two versions in addition to HTML.

[codecov-commenter]

Codecov Report

Merging #240 (ea0e451) into main (8dcf6a1) will not change coverage.
The diff coverage is n/a.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

@@           Coverage Diff           @@
##             main     #240   +/-   ##
=======================================
  Coverage   88.54%   88.54%           
=======================================
  Files           4        4           
  Lines         978      978           
  Branches      203      203           
=======================================
  Hits          866      866           
  Misses         82       82           
  Partials       30       30           

Flag	Coverage Δ
unittests-3.10	88.54% <ø> (ø)
unittests-3.6	88.21% <ø> (ø)
unittests-3.7	88.21% <ø> (ø)
unittests-3.8	88.34% <ø> (ø)
unittests-3.9	88.34% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.