BUG: Docs build fails with sphinx 8.2.0

[connorjward]

Since the release of Sphinx 8.2.0 (and the subsequent release of our docdeps container) docs have been failing to build with the error message:

Traceback
=========

      File "/home/firedrake/firedrake/lib/python3.12/site-packages/sphinxcontrib/inkscapeconverter.py", line 71, in convert
        args += ['--export-filename=' + _to, _from]
                 ~~~~~~~~~~~~~~~~~~~~~^~~~~
    TypeError: can only concatenate str (not "_StrPath") to str

We should pin to <8.2.0 for now and report this issue upstream.