Language support in User Guide

[imnasnainaec]

https://squidfunk.github.io/mkdocs-material/setup/changing-the-language/
squidfunk/mkdocs-material#2346
https://ep2021.europython.eu/media/conference/slides/8FcL3BL-create-beautiful-and-localized-documentations-and-websites-usi_dlPBzbu.pdf
https://www.mkdocs.org/user-guide/installation/
https://www.privacyguides.org/en/meta/translations/#admonitions
https://ultrabug.github.io/mkdocs-static-i18n/

We want the docs to support the same languages as The Combine.

Crowdin supports Markdown so we uploaded the user guide.

• introduce an en folder for our English docs ([User Guide] Reorganize to prepare for other languages #2354) (retracting for a different approach)
• add a the language picker ([User Guide] Add Spanish (es) #2716)