Documentation reorganization

[pet-mit]

This PR includes a reorganization of the documentation content structure, plus some extra content:

• The website now has a richer tree, most of the content was redispatched in the new pages:
  • the top-most categories are "user guide" (instead of "reference guide") and "developer guide" (instead of "build")
  • the "user guide" is now easier to read:
    • with a focus on Antares_Simulator's two main features: solver and ts-generator
    • the other features are briefly documented but marked as deprecated
    • each of the features now has a new "parameters" page
    • the solver now has a new "heuristics" page intended to document all the heuristics used by the solver
    • both features have "inputs, outputs and command line" pages
• The PDF generation action is now run on same branches as other actions, in order to check that the PDF builds correctly
• New README in the docs directory to explain how developers should maintain the docs
• Some overall updates & harmonization
• There is still a lot of work to be done (at least the tasks marked with a "TODO" comment)

The resulting doc can be previewed in the readthedocs automatic check or here: https://antares-simulator--2024.org.readthedocs.build/en/2024/