Add documentation for delete by query plugin

[tlrx]

Add documentation for delete by query plugin (see #11516). This page is placed in a /plugins directory until we figure where to place all plugins documentation.

[dadoonet]

@tlrx I made a proposal here elastic/elasticsearch-analysis-kuromoji#62 about plugin (module) documentation.
I think it should goes into src/main/asciidoc to meet maven convention for asciidoc docs.

So I'd make that consistent. In maven world, source files (code, documentation, resources...) should always be in src dir.

Would love to get @clintongormley thoughts about this so I can move my PR from the plugin repo to elasticsearch repo.

[dadoonet]

I would prefix with plugins- to be consistent with the very first tag.
Same for others.

[tlrx]

@clintongormley I just updated with your modification, thanks for your review.

@dadoonet I think I created too many sub sections :) Thanks for your review by the way.

[clintongormley]

LGTM - we'll need to change the << >> links when we move it into the plugin docs, but we can do that then.