Discuss: Auto-merge `auto generated update of versioned plugin documentation` on clean docs-ci (and delete branch)

Pros:
* Keeps plugin docs in [Versioned Plugin Reference (VPR)](https://www.elastic.co/guide/en/logstash-versioned-plugins/current/index.html) up-to-date without manual review and merge.

Neutral:
* Failing docs CI still would require manual troubleshooting and fixes to both output and source files.

Cons:
* This is the last line of defense where I verify that plugin docs are of sufficient quality before they're merged.
Ideally, plugin docs have been throughly reviewed before PRs are merged, but a lot of less-than-ideal wording and formatting still gets through. Some of those things could be mitigated with improved CI/linking on the plugin repos as proposed in #84.

ToDo:
* [ ] Make sure that the new content branch is deleted after merge to keep the docgen/PR process going.
* [ ] Come up with some kind of [somewhat loud] alerting mechanism when there is a docs-ci failure.