all: lint markdown files

[l-zeuch]

Add and configure a markdown linter. While we're at it, also fix the
lints it threw, excluding select files (especially all-commands).

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This is a combination of two commits. Review should not be required,
though it may be useful to review at least the first commit.

• workflows: add and configure a Markdown linter
• all: lint markdown files

Terms

• I have read and understood this project's Contributing Guidelines

[l-zeuch]

Everything works as expected (though I did test it locally with act,
one can never be too sure). Merging as-is.