Add documentation for use of the webhooks for triggering materials

There appears to be no consolidated guide/howto for use of the SCM notification webhooks. While they are included in [the API docs](https://api.gocd.org/current/#webhook) these aren't really traditional API and seem they should be documented in the main documentation.

Suggested content
- at least with links to the API docs to avoid repeating ourselves
- explain why you might want to use this (it's often a suggestions to alleviate polling load/throttling on target VCS systems and take load off the material subsystem threads (and even H2) so it'd be nice to have something to point to
- link from [Configuration / Add Material](https://docs.gocd.org/current/configuration/admin_add_material.html) perhaps?
- As suggested at https://github.com/gocd/gocd/pull/10091#issuecomment-1027859107 perhaps it should also note how to disable polling on materials.
- the `config_repos` webhooks added in `21.1.0` in #8023 seem to not be documented at all; this documentation should probably include mention of that, or build on top of https://github.com/gocd/api.go.cd/issues/1037