Logstash - Support parallelism in Plugin Reference Docs generation logic.

## Issue description
We had a bug in Plugin Reference Docs generation logic. The race condition in integration embedded plugins and plain plugins brought an issue where docs will not be generated or strangely generated.
During my spacetime, I quickly tested single thread and applied cash (in case), raised a [PR](https://github.com/elastic/docs-tools/pull/81) to get to know how team wants. The conversation ended with separating short & long term solution. The PR covers the short term (uses single thread) and for the long term we have to separate the logic to first generate the docs for integration plugins, then by excluding them we should proceed the plain plugins (of `plugin_versions.json`).

## Consider the importance
Do we really need this? - so far we are not in really need of this feature but have a nice feature since VPR is already using parallel processing.

## Acceptance Criteria
Doc generation logic should support number of threads to speed up the doc generation process.