Pdf is not built on merges to main

[samcunliffe]

Describe the bug
The pdf documentation should be rebuilt on merges to main or on touching the LaTeX source. The last build on main was ages ago.

Expected behaviour
Should be rebuilt and published on every push, like the Doxygen docs.

Additional context
We could consider uploading as an artifact and committing it to the gh-pages branch along with the doxygen, perhaps in _assets? Then we can do away with the (currently stale) gh-pages branch completely.