Redesign of the docs

[astrofle]

This PR changes the outline of the documentation, splitting the contents into four main categories: tutorials, how-to guides, explanation material and reference material.

One downside of the current implementation is that the example notebooks are symlinked twice, one for tutorials and one for how-to guides, which means all notebooks get executed twice 👎

To try it out (assuming you have sphinx>=7.3). From the dysh root dir:

sphinx-build docs/source docs/build -b html --fresh-env
sphinx-autobuild docs/source docs/build -b html --host=`hostname` --port=<xxxx>

then navigate to `hostname`:<xxxx> to see the docs (assuming you are in the same network as `hostname`).

I'd also recommend changing this line in docs/source/conf.py for testing (don't commit that change. I do not know what that would do to the ReadTheDocs build):

nb_execution_mode = "cache"

to

nb_execution_mode = "auto"

this avoids executing the notebooks multiple times.

If you find anything missing or any errors, please leave a comment/review.