Add URL of documentation on docs.github.com/webhooks

[gr2m]

It would be useful for when generating docs such as https://github.com/octokit/webhooks.js#webhook-events. Not all events are listed on https://developer.github.com/v3/activity/events/types/#checkrunevent, ping and repository_dispatch are documented in other places

[gr2m]

@wolfy1339 I think this can be closed? The schemas all seem to contain a URL in the description, e.g.

webhooks/payload-schemas/schemas/check_run/completed.schema.json

Line 9 in 1b72978

"description": "The [check_run](https://docs.github.com/en/rest/reference/checks#get-a-check-run).",

[wolfy1339]

That would be the description for the check_run object within the payload.
They don't all contain links at their root to the docs