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

See https://github.com/crickets-and-comb/shared/issues/52

Start by reading the linked Sphinx internationalization guide and the example workflow, then inspect the related issue crickets-and-comb/shared#52 for project context. Clarify which documentation languages and translation workflow are required; done should mean the documentation supports the agreed multilingual setup using sphinx-intl.

Written by the indexing model from the issue text.