Document media use

[CecSve]

also document what the link below an image in an occurrence record is populated. Currently, we have one identifier field in the occurrence media table, but multiple sources can be published by the publisher if they use extensions, so it is unclear what happens behind the scenes for populating the field.

Originally posted by @CecSve in #92

[MattBlissett]

An occurrence with several multimedia 'objects', whether from an extension or the pipe-separated dwc:associatedMedia term, would get several rows in the occurrence multimedia table / multimedia extension in a download.

[CecSve]

documentation initiated: https://github.com/gbif/tech-docs/blob/main/en/data-use/modules/ROOT/pages/multimedia-use.adoc