Check links and references

[Fabien Maussion]

After the consequent refactoring of #114 it would be good to go back to the following checks:

1. warnings when building the book. They all come from the fact that we use "hacks" for internal references within a notebook while jupyter-book uses myst syntax
2. linkcheck.sh checks that all our links are working (some don't)

Both can be done relatively quickly by changing notebook exectution to "off" in config.yaml. (1) is more problematic because I don't know a way to do internal references in both standard markdown and myst markdown (used by jupyter-book). See also: https://github.com/orgs/executablebooks/discussions/1129