Add documentation on fediverse:creator tag

Currently, this is only mentioned once, under `attributionDomains` in the ActivityPub reference documentation. - how to add this on a web page - how to specify / verify domains in the user account settings - how to check / clarify setup More broadly, we also need to expand and add information on how to integrate Mastodon elements with web sites e.g. how to link to a Mastodon profile, how to embed Mastodon posts, etc.