[Checks] lint-md checks _build unnecessarily

I noticed when cleaning up some lint-md violations in my docs that the lint-md automated check also checks the output of _build. Since the _build dir doesn't contain any markdown, this seems unnecessary and wasteful. By default this should be skipped, or instructions on how to skip it should be added to the docs.

Edit: if this is checking the pages generated by llms.txt it should still be skipped, as these are not pages we can reliably control the content of (they're continuously overwritten each time the docs are rebuilt).