Make the documentation truly multilingual

[mlegner]

Description

#154 adds the basic tools to offer this documentation in multiple languages. There is some follow-up work to make this truly multilingual:

• Add a language selector to the final book. See this for reference.
• Build all languages in our CI workflows and include them in the published book. See this for reference.
• Add disclaimer on all translations that they are best-effort and (largely) maintained by the community.
• Find a way to support translations of admonitions.
• Make sure the message templates are updated for all changes to the docs.

[lispking]

This is a great issue. I've noticed that many books also include a navigation bar. After the Chinese version is merged, I will add one.

[lispking]

@mlegner pls assign to me, thanks.