docs: Rewrite portion of Provider spec

Plus addressing of a couple of nits.

The following must still be picked up at a later moment:

- More sections need to be moved from "working with" to "writing a ..."
- Documentation flow can likely be improved once the sections have been
  moved.
- Technical description of the behavior of the Provider types could be
  improved, this should be easier to do when everything has the same
  format.

Signed-off-by: Hidde Beydals <[email protected]>

Author: hiddeco

docs/spec/README.md

@@ -36,7 +36,7 @@ Notification API:
 
 * [Alerts](v1beta2/alerts.md)
 * [Providers](v1beta2/providers.md)
-* [Git Commit Status Updates](v1beta2/statusupdates.md)
+* [Events](v1beta2/events.md)
 
 The alert delivery method is **at-most once** with a timeout of 15 seconds.
 The controller performs automatic retries for connection errors and 500-range response code.