DOC: Migrate to Read the Docs configuration file v2

[jhlegarreta]

Migrate to Read the Docs configuration file v2: add a .readthedocs.yaml config file to configure the documentation build process.

Documentation:
https://blog.readthedocs.com/migrate-configuration-v2/

[jhlegarreta]

Build failures are unrelated. Adjustments and fixes to the config file will be made as failures are detected. Merging.

[jhlegarreta]

Building the documentation is failing:
https://readthedocs.org/projects/tractolearn/builds/21088860/

The error being:

  File "/tmp/pip-install-6qy2spnr/python-spams_ad368899a0284d1cbcbc3f153a0fb148/setup.py", line 29, in get_config
    for _ in np.__config__.blas_opt_info.get('extra_compile_args', []):
AttributeError: module 'numpy.__config__' has no attribute 'blas_opt_info'
[end of output]

At least it is linked to issues encountered in PR #59 and the build failure in the current main:
https://github.com/scil-vital/tractolearn/actions/runs/5346105673/jobs/9692646941#step:5:937

So fixing it is a first necessary step towards getting the documentation build fixed.