**Motivation**: We have decided to rewrite the documentation generator to add more features
to our documentation and to fix some outstanding bugs.
This PR addresses the following problem:
- Hard to add new features to the code base.
- The code was tied with Logstash.
- No easy way to publish the doc for a single plugin
- The plugin author was not able to test their documentation changes
- The reported errors were hard to understand.
- No easy way to automate it.
- the minimal requirement on base classes.
Fixes#5720
