add lighthouse audit action and language in docs/conf.py

[teddygroves]

The main change here is to add automatic accessibility checking using the lighthouse check github action.

This should make it easier to keep track of whether bibat's documentation is accessible as suggested here.

I tried to emulate the approach taken by pydata-sphinx-theme.

Checklist:

• No pytest errors
• make analysis works
• README.md up to date
• docs up to date
• {{cookiecutter.repo_name}}/README.md up to date
• investigate.ipynb up to date

[codecov]

Codecov Report

Merging #61 (c191248) into main (2b3bbc4) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main       #61   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           64        64           
=========================================
  Hits            64        64