docs: migrate to markdown/ mkdocs/ material

[lkstrp]

Closes #2059

Changes proposed in this Pull Request

• I still went with Material and MkDocs (same setup as in PyPSA), but once Zensical is stable we should move there. It currently does not support all features, but it will be close to a drop-in replacement for MkDocs. I cannot find the article right now, but the MkDocs and Material for MkDocs maintainers are not very aligned right now. Another migration will be much easier though, since we only need to touch plugins then. EDIT (https://fpgmaas.com/blog/collapse-of-mkdocs/ Thanks @euronion)
• All features previously using Sphinx are moved over. For the yaml stuff I had to write a macro, and all DAGs are now read from files instead of being created during the build.
• Should we also move this to a new URL? Either docs.pypsa.org/pypsa-eur or its own subdomain?
• To review this, the commit after the rename should be checked. There we still get a proper diff: 5634986
• If downstream repos changed the documentation, this will be a bit of a nightmare to pull in. It will be easiest to just apply the full downstream diff again on top of here. I think I saw separate docs for e.g. TYNDP @daniel-rdt @tgilon
• @fneum I will do a couple more changes and also go through the full diff again, but you are already free to push whatever on here.

Run this locally with pixi run update-dags & pixi run -e doc mkdocs serve
Find the build here

This should again not be squashed. We lose the git history otherwise.

Checklist

• Another check of everything, to find all hallucinations I missed
• Some Design Tweaks, haven't touched that yet.

[daniel-rdt]

Thank you @lkstrp for the heads up!

[fneum]

Looks great! I would merge that ASAP and then built improvements on top. Needless to say, the documentation of PyPSA-Eur is in a shocking state.

• YAML blocks: The YAML selection did not always work correctly (often to long, esp. with shared setting endings or duplicate keys like "enable"). Must be changed before merge.

• Configuration: There's a mistake in how indentation levels >= 2 are treated. They are shown on the same level as the first. The "renewable" section does not look right (duplication and notes inception). Some unresolved literalinclude and jsonschema RST syntax. Must be changed before merge.

• Release Notes: Can we get hyperlinks for PR numbers? Should be changed before merge.

• Rules: Some unresolved .. code:: yaml and .. tip:: blocks. Should be changed before merge.

• Data Sources: Links in table are not clickable. Can be changed before merge.

• I have ideas for tabs Home, Design, Configuration, Rules, Contributing. To be tackled later.

Everything else, later -- I would say.

[fneum]

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[fneum]

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[fneum]

Also enable: section in configuration.