Support localizing the menus

Fixes #396

:warning: the localization are not done in this PR per se.

This PR add the following commits:

Translate the sections of the left menu which:
- introduces help/{en,fr}/sections-translations.yml which will contain translations for the sections (items in the menu that contain children). GitLocalize is able to translate the keys (see screenshot below). These files are empty, a PR is expected to be automatically open once this PR is merged.
- Loads the sections-translations.yml file to change the section titles using the mkdocs on_nav hook
Move titles in documents metadata: adds the titles of the pages in documents themselves, so they are translated along with their content in Git Localize
Remove titles from mkdocs.yml, they are now in the docs metadata: Each doc now embeds metadata containing the title. This way, the titles can be localized when consulting a document.
Add github workflow to automatically run ./docs.py update-languages: This introduces a job that automatically run update-language, and if it changes some files, automatically open a PR, especially useful for updating the sections-translations.yml files.

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gristlabs / grist-help