hi Yann1ck, thank you for your interest.
You raise good questions. I suggest the Refactor the project documentation initiative post to continue general discussion on the doc refactoring. Let's try to keep this one for joplin-docs.
I used Grav because I knew it and saw an opportunity for a quick start. A solution was almost build already. I will have a look at GitBook, MkDocs, thank you!
For me Grav is a light cms (maybe due to my Drupal & WordPress background) and I like how the content is managed (one folder by page to keep resources grouped).
Even if Grav is not the best choice, we can start building a better doc today using it (Importing the doc in the final solution should not be a big deal as content is .md files & folders).
A separate repo makes sense for me (as it allow quick merges, no risk to mess with the app, can evolve separately). If using xaa/joplin-docs is a concern (to stay in the "Joplin sphere"), Laurent could create the repo then I plug Grav on it.
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Regarding your question about the FAQ:
At this stage I see it as temporary section where we can add the relevant posts from the forum pending the new doc structure is defined and approved. A complementary section living close to the general doc.