Change "Media File" to "Form Attachment" or "Submission Attachment"

We're making this change in the Central frontend for v2022.3, so we should make this change in the docs as well. "Media File" should be replaced with either "Form Attachment" or "Submission Attachment", or (if the context is clear) just "Attachment".

Central frontend PR: getodk/central-frontend#659

Search the documentation for “Media File” and review each occurrence in the surrounding context. Use the Central frontend PR getodk/central-frontend#659 as the terminology reference, replacing the wording with “Form Attachment,” “Submission Attachment,” or “Attachment” where the context is clear; done means the relevant documentation no longer uses the old term.

Written by the indexing model from the issue text.