- Add the document title to the suggested filename for exports, to make
   it unique across different documents by default.

 - Move `export-annotations.ts` to the `helpers/` directory, since it
   contains functions that are very much application-specific. The
   `util/` directory contains code which is potentially usable in other
   applications or libraries / is less domain specific.

 - Add a note about how filenames that are invalid on the user's system
   are handled

Part of https://github.com/hypothesis/client/issues/5709