Allow list of files to be passed to lint-doc.sh

What does this MR do and why?

Allow list of files to be passed to lint-doc.sh. This supports local tooling where you only want a subset of files to be markdownlint / vale checked. Both of these can take a lot of time to run locally.

This only modifies calls to markdownlint and vale (as opposed to passing a file list to all the other checks). This is because those other checks run very fast. So just making a small iteration to focus on the long running checks.

How to set up and validate locally

Edit the file doc/administration/instance_limits.md, and add this sentence:

Allow list of files to be passed to `lint-doc.sh`.  This supports markdown

Add the same sentence to doc/api/access_requests.md. This generate errors for both markdownlint and vale.

Run the following commands, and you should see the same error detected:

• scripts/lint-doc.sh doc/administration/instance_limits.md doc/api/access_requests.md
• scripts/lint-doc.sh

MR acceptance checklist

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