Add docs translation and the infrastructure and automation it requires

[CAM-Gerlach]

Pull Request

Pull Request Checklist

• Read and followed this repo's Contributing Guidelines
• Based your PR on the latest version of the correct branch (master or 4.x)
• Checked your writing carefully for correct English spelling, grammar, etc
• Described your changes and the motivation for them below

Description of Changes

As a followup to #377 and #379 , add the actual infrastructure and automation tooling for translating the Spyder documentation, and the translations themselves.

Specifically, this adds:

• The actual pot and po files with the translations, pulled from the versions in Crowdin
• Commands for generating and updating the translation files
• Commands for building the docs with multiple translations
• CI support for the same
• Crowdin configuration for future automatated pulls and pushes of translation updates

The site is now built into the translated versions; to view this, add LANGCODE/ to the URL after the version, currently either en/ or es/. There is not yet a GUI switcher for translations; the current translations can be switched by manually editing the URL. Additionally, the translations need some additional work to translate untranslated texts (overview, FAQ titles, captions, etc) and update the translation to be generated with the current documentation rather than one several months old. Therefore, it will remain beta for now.

[CAM-Gerlach]

Now updated the PR by:

• Making the build-multilanguage command build both the base docs and the en and es locales (to fix the missing docs in the site root)
• Adding a new build-languages command to build just the (specified or default) language(s) (previous behavior of build-multilanguage)
• Modifying the update-po command to just update the en po by default (as we need with Crowdin, and for the forthcoming auto-update job)
• Adding an --all-languages flag to restore the previous behavior of building all source and target languages, which now also supports specifying additional languages,
• Adding a build-pot-po file to build new pot files, update the existing stored ones and also the en po-files from them, to allow updating the source translations in one command and to be used for the forthcoming auto-update job

[ccordoba12]

Great work @CAM-Gerlach!

The final step to publish our Spanish translation is getting closer and closer!

[CAM-Gerlach]

In fact, it's already technically published with this PR; we're just missing up to date/complete translations (which I can kick off shortly), and an easy way for people to switch between translations (the switcher).