doc: remove dune:action directive

[emillon]

The goal of this PR is to remove the custom python code to handle dune:
directives.

This plugin (in doc/exts/sphinx_dunedomain.py) defines directives that do several things:

• they generate nice markup that describe what we're documenting (the headers with blue or gray backgroud)
• they support cross-references
• they generate an index

But, this is too complicated for our use case:

• cross-references are unnecessary now that we have one document per action
• the index was not linked, but we now have a nicer one in reference/actions/index
• crucially, there's a describe directive in Sphinx that generates the same markup as the one we were generating.

So this PR does 2 things:

• switch from the high-level dune:action:: to a low-level describe:: (which renders identically)
• remove the now unused support code in sphinx_dunedomain.py

[emillon]

(as followup PRs I have similar ones that do the same for dune:field and dune:stanza, thus removing sphinx_dunedomain completely)

[ElectreAAS]

While this looks obviously correct, and I'm generally in favor of removing python code, do you know more about the why was this code added there in the first place? And what do you mean by unneeded cross-references here?

[emillon]

I wrote the python code in question. At the time I thought that this was the only way of getting this kind of markup (I didn't know about describe::). Indices and cross-references are nice sphinx features, but they're not really required: instead of using a xref to link to the cat action, we can just link to the cat document (now that it exists).

[emillon]

Thanks!