docs: Markdown linting

[cpswan]

As this is a docs repo we should ensure that the content is of a good standard

- What I did

• Linted existing files with pymarkdownlnt
• Created automation to check future PRs

- How I did it

• Installed pymarkdownlnt locally in a Python venv
• Worked through existing files
  • Identified need to ignore long lines (MD013), which we have a lot of in tables
  • Identified need to ignore Multiple headings cannot contain the same content (MD024) as it makes sense how we're doing that
• Copied/modified existing workflows from at_server and at_demos

- How to verify it

The (lack of) changes should be visible in rich diff.

Subsequent PRs will run the new workflow.

- Description for the changelog

docs: Markdown linting