Doc: LS Versioned Plugin Reference does not display (or hide) conditional install instructions for plugins

Conditional processing is set up in the Logstash Reference (LSR), but is not implemented for the VPR.

#### Details
Some plugins are installed with Logstash by default, and others are not.
For plugins that are not installed by default, our LSR docs include installation instructions.
The settings and tooling are not implemented for the VPR.

* `Default_plugin` is managed here for LSR docgen: https://github.com/elastic/logstash/blob/main/rakelib/plugins-metadata.json
* The `default_plugin` value appears at the top of the generated plugin doc.
* Conditional statements that control whether install content is shown or not is here: