doc: flesh out user credential documentation

Created by: LawnGnome

This PR adds documentation for creating and removing user credentials in Sourcegraph. I've also tried to plumb this through where it makes sense in the campaigns documentation, but I'm sure I've missed places.

The rendered documentation is at https://docs.sourcegraph.com/@aharvey-user-token-docs/. The most interesting page is probably the new https://docs.sourcegraph.com/@aharvey-user-token-docs/campaigns/how-tos/configuring_user_credentials, which is full of screenshots and videos. (Well, OK, the same video twice. Close enough.)

Fixes #15312 (closed).