Rebuild files that contain diagnostics

[alcarney]

What would you like to see

Some diagnostics can be fixed by making changes to another file e.g. writing :ref:`not-a-valid-ref-yet` , and then switching to another file and defining not-a-valid-ref-yet.

However since the original file is not rebuilt (as no changes have been made), the stale diagnostic persists. If the sphinx-agent rebuilt the file automatically, then the diagnostic would automatically be cleaned up.