We redirect existing URLs to the new URLs. We want to use
`/integrations/category/{category_slug}` for categories and
`/integrations/{integration_name}` instead of
`/integrations/doc/{integration_name}` for individual integrations page.
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.
