Enable rstcheck in CI and lint code blocks

[ml-evs]

This PR follows up on #493 by following GitHub's advice of running the spec through rstcheck. We were already passing it, but in the process of adding it I think it makes sense to properly lint our code blocks as JSON data (which is possible in many cases without losing the structure of them).

Also this PR:

• unifies the indent of all code blocks to start after the 3 chars indicated with the RST ".. ".
• fixes one occurrence of a non-standard field in a code block ("response_message" for the meta example)
• fixes indentation of one implementation note so that it refers to its proper field
• switches the partial data code blocks to JSONL format

Unfortunately its a bit tricky to see how this will render in the future in GitHub, so perhaps we need to hold off a little while for the official response.

[rartino]

@ml-evs This is marked as a draft, but is it not good to go now? The GitHub pre-commit checks pass and seem to include rstcheck.

(That said, in some future cleanup I would like to just float if it is possible to move these checks into 'make audit' instead, and then only have a single 'make audit' in the pre-commit hooks. That makes it a bit more straightforward to keep the pre-commit hooks aligned with the checks it is easy to do oneself when working on a clone, and it would also avoid us becoming unnecessarily dependent on git-specific features.)

[ml-evs]

In that case, feel free to review and we can merge here. Otherwise also happy to use make audit straight away. There might be some conflicts here with #499 -- perhaps this should be merged first as it introduces additional checks.