Make docs multilingual with `sphinx-intl`.

We can use the `sphinx-intl` extension to make our docs multilingual. https://www.sphinx-doc.org/en/master/usage/advanced/intl.html

Here's an example of how to use it: https://www.codingwiththomas.com/blog/my-sphinx-best-practice-for-a-multiversion-documentation-in-different-languages

Start with the linked Sphinx internationalization guide and the example workflow, then inspect how this repository currently builds its documentation. Define the supported languages and translation workflow; done means the documentation can be built and maintained in multiple languages.

Written by the indexing model from the issue text.