DOC: ensure examples in documentation are not skipped

[bsipocz]

Some of the MIVOT/MANGO documentation examples are skipped at the moment.

I would be preferable if we would test all the examples we include in the narrative docs to ensure what we show to end users actually keep working.

The only exceptions are the extremely resource intensive examples like running a query that is way too wide scoped, we have a few of those in the registry docs. For any such cases we should aim to provide a comment line in the docs source code so the reason for the skip is easy to see.

I'm labeling it as mivot, but there are a couple more cases in docs/dal/index.rst, too, and maybe there are still code snippets that are not in the doctest format and thus are always skipped.

Tasks:

• look for doctest-skip directives, endure each have a reason stated in a comment, remove the skip from as many as reasonably possible
• look for code-block directives, add >>> to them so they can be picked up by the testing infra