events: add categories and other less documented event types

[xvaier]

[aviau]

We already document categories elsewhere in the docs:

• https://api.docs.flare.io/advanced/event-source-filters

We don't want to document at two places.

[xvaier]

We already document categories elsewhere in the docs:

* https://api.docs.flare.io/advanced/event-source-filters

We don't want to document at two places.

Ah, I knew I had seen this table, I changed it to be a link to the original page.