split versioned plugin doc generation ci job into two

currently the ci job to generate docs performs the following tasks:
1. generates docs for all plugins
2. clones docs repo and tests generated plugin docs
3. creates creates a PR if generation is sucessfull

The problem with this approach is that, [if there's an error with the docs, the PR is never created](https://logstash-ci.elastic.co/job/elastic+docs-tools+master+logstash-versioned-plugin-docs/34/console) so there's no easy way to work on the broken PR to make it work. So instead we should have two ci jobs:

1. One that periodically computes new docs and creates a PR
2. Another that takes a PR and tests against the docs repository