chore: lint markdown new-features.md #15149

[gianarb]

Reference #15149

Motivation

Currently we do not lint the new-features.md file because it fails
straight during a release.

Modifications

This pR makes the new-features.md compliant with the markdownlint
tool and it adds linting check at the right time and not during a CI
release.

I had to fix a few .features/pending files that could not be combined into a
new-features.md without making the linter to go crazy.

And then I added markdownlint in a few more places in the Makefile.

Verification

make features-update should work with no issue and lint the generated file.

Documentation

[Joibel]

We need new features not to make it into the repo with these failures too.

[Joibel]

I think people who're developing a new feature would expect to be able to run make features-validate and have it tell them if it would fail linting - but currently this won't.

[gianarb]

I thought about the same, but I see a bit of a race condition here because in order to run the mdcheck we need to actually have a file and not print to stdout. This makes the features-validate very similar to features-update one. But if we merge those it means that the user running features-update in order to run correctly needs to know the VERSION to set, and I think it is not what we want.

Another option I thought about is to change the features-validate target to temporarily write into a file somewhere, mdcheck and remove the file. In this way it is "okey" if the generated output does not have the correct version but the default one (latest).

[Joibel]

I don't really mind how we achieve this, the validate target's only job is to validate the markdown - and fixing it so VERSION doesn't matter is fine. Be as brutal as needed. If it effectively runs update, that's also fine - perhaps we can avoid a temp file by piping the output to markdown lint?

[Joibel]

Awesome, thanks