programatically generate plugin-type asciidocs

[jsvd]

The asciidoc index documents for inputs, filters, codecs and outputs are currently manually edited.
If a plugin is removed from the documentation, it's necessary to delete the references from the index. If a plugin is added, the include must be added as well.

This process should be automatic during the asciidoc fetch/generation.

depends on elastic/logstash#6086

more information on elastic/logstash-docs#422 (comment)