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