Docs: Explicitly specify English language to Sphinx

[Lestropie]

Addressing CI error observed in #2713:

Run python3 -m sphinx -n -N -W -w sphinx.log docs/ tmp/
  python3 -m sphinx -n -N -W -w sphinx.log docs/ tmp/
  shell: /usr/bin/bash -e {0}
  env:
    QT_SELECT: qt5
Running Sphinx v7.[2](https://github.com/MRtrix3/mrtrix3/actions/runs/6147387227/job/16678768734?pr=2713#step:7:2).[5](https://github.com/MRtrix3/mrtrix3/actions/runs/6147387227/job/16678768734?pr=2713#step:7:5)

Warning, treated as error:
Invalid configuration value found: 'language = None'. Update your configuration to a valid language code. Falling back to 'en' (English).
Error: Process completed with exit code 2.

[Lestropie]

Looks to be fixing CI on master if someone in @MRtrix3/mrtrix3-devs is happy to review.