Disable PDF docs build

[glatterf42]

As noted in #734, until we decide/invest enough time to enable the PDF builds for our docs (again), we should remove this version of the docs to keep the builds from failing.

How to review

I don't think we'll be able to build the PDF version from a PR build, we might be able to trigger a run manually on RTD for a branch build that builds epub and html, but no pdf. If not, we will only see this working after the merge, but the changes are small enough to understand they won't break anything.

• Read the diff and note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• [ ] Add or expand tests; coverage checks both ✅ ONly change docs config.
• [ ] Add, expand, or update documentation. Only change docs config.
• [ ] Update release notes. Only change docs config.

[codecov]

Codecov Report

Merging #735 (d928f98) into main (e748b8c) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #735   +/-   ##
=====================================
  Coverage   94.5%   94.5%           
=====================================
  Files         43      43           
  Lines       3463    3463           
=====================================
  Hits        3273    3273           
  Misses       190     190           

[khaeru]

Thanks. Let's update the description of #734 to indicate that it is a placeholder/record of some experimentation and attempts; that this current PR was the action taken for now; and that the attempts could be continued if that was judged high(er) priority.