Improve documentation

[benbovy]

Documentation was largely outdated.

Checklist:

• [x] Convert rst to md (MyST) except for API docs
• [x] C++ docstrings and API documentation
• [x] Python docstrings and API documentation
• [x] Python type hints
• [x] mypy github action
• [x] check links in md files github action
• [x] why fastscapelib section
• [x] High-level examples (notebooks run when building the docs)
  • [x] Basic simulations with Python
  • [x] Basic simulations with C++ (if xeus-cling works with Myst-NB on RTD)
• [x] User guide with C++ / Python code blocs (tabs)
  • [x] Grids
  • [x] Flow routing
  • [x] Eroders
• [x] Fastscapelib internals (grid CRTP, flow operators, etc.)
• [x] Add related publications page (+ internal links from docstrings / docs)
• [x] Update Install / Build instructions
• [x] Update README and Documentation index
• [x] Contributor guide
• [x] Update RTD config
  • [x] Enable PR builds
• [x] Add logo

[benbovy]

@hannahsophiadavies FYI I just finished rewriting the documentation from scratch.

This pull-request is way too big to review, so I'm going to merge it and go ahead with other items in the roadmap. However, it would be great if you can have a look at it whenever you have some time. I would greatly appreciate any feedback, ideas or suggestions to improve the documentation! (this could be done in new issues)

The rendered documentation can temporarily be viewed here: https://fastscapelib--120.org.readthedocs.build/en/120/

After this PR is merged, the new documentation will be here: https://fastscapelib.readthedocs.io/en/latest/