Updates text of generate-webhook-url-basic.md to replace the use of the "Create the channel" step. Removes {!create-channel.md!} from all the integrations docs that also include {!generate-webhook-url-basic.md!}. Fixes #32269.
Part of zulip#29592.
Creates an incoming webhook integration for Github Sponsors. The main use case is getting notifications when new sponsors sign up. Fixes #18320.