sphinxdocs: i18n support

# sphinxdocs: i18n support

### Relevant Rules

`sphinx_docs` in `//sphinxdocs/sphinxdocs/sphinx.bzl`

### Description

`sphinx_docs` should accept a `lang` array of strings and produce docs for each specified language

### Describe the solution you'd like

per https://www.sphinx-doc.org/en/master/usage/advanced/intl.html I think it would have to run `sphinx-build gettext`, save the `.po` files somewhere within the repo (pretty sure that these are checked into the VCS), and then run `sphinx-build -Dlanguage=` for every specified lang

### Describe alternatives you've considered

`sphinx-intl` seems to be the common extension for i18n workflows but it doesn't seem strictly required https://www.sphinx-doc.org/en/master/usage/advanced/intl.html#translating-with-sphinx-intl

Start by reading the `sphinx_docs` rule in `//sphinxdocs/sphinxdocs/sphinx.bzl` and the linked Sphinx internationalization workflow. Done means the rule accepts a `lang` array, generates translation files as needed, and produces documentation for every specified language.

Written by the indexing model from the issue text.