[CI] Validation

[novusnota]

Like "linter", but can come up with things that won't be autofixed and require manual intervention. It might be named "auditor" or "audition" too.

It's worth noting that remark-lint, the linter based on the remark parser, cannot fix the found lints itself, so we ought to write a simple script that will be able to do so. It might be made on top of remark-lint, in which case we'll need to apply fix options to each lint carefully; otherwise, implement each lint ourselves, not a big deal with having remark around.

Validation can be applied:

• to .mdx files
• to other file types
• to issues, with related scripts going in a separate workflow that spawns for issue actions