[WIP] Add configuration files for Sphinx/ReadTheDocs

[JF002]

Following our discussion in #748, I open this PR to start working on the re-working of the documentation of the project:

• The documentation (user documentation, developers documentation,...) is stored in this repo
• It's processed by the documentation generator Sphinx and hosted on ReadTheDocs.
• The URL https://docs.infinitime.io will point to the documentation on RTD (Done)

I created a new team doc-writer for contributors who focus on writing and maintaining the documentation of the project. I also invited @ncartron to join the project as the first member of this team.

The 'new' doc is in the folder docs while the old one is still untouched in doc. Ultimately, we will keep only one doc* folder.

NOTE : @Riksu9000 has already created a PR about the documentation (#810 ). It's certainly a good idea to take those changes into account! Let's review this PR asap!

[JF002]

@ncartron A new PR was recently created with new improvements to the documentation we might want to integrate in the new documentation : #854

[ncartron]

@JF002 got it, I'll look at #854 and will integrate what makes sense in the new doc.

[ncartron]

Alright, I reviewed the changes in #854 and they are mostly in the dev part - I'll incorporate them once I am working on that part of the new documentation (I am still in the user documentation ATM).

[github-actions]

Build size and comparison to main:

Section	Size	Difference
text	409300B	0B
data	996B	0B
bss	63372B	0B