Encourage doc authors to also edit help_links.js

[JohnVeness]

I've noticed that several of the in-app help links (https://github.com/frappe/erpnext/blob/develop/erpnext/public/js/help_links.js) are wrong, presumably because files are renamed or moved in the folder hierarchy.

In the documentation for how to write the documentation, maybe add a note to suggest that help_links.js should be kept in sync when files are created/deleted/renamed?