Fix asciidoc link to string reference

Fixes #2038
More asciidoc fixes for base input and output

Fixes #2038
Fix formatting for word plain

Fixes #2038
Changes to generated docs and their templates.
Handling feedback for PR#2041
Merge branch 'erbdocs' of https://github.com/palecur/logstash into erbdocs

Conflicts:
	docs/index-codecs.asciidoc.erb
	docs/index-filters.asciidoc.erb
	docs/plugin-doc.html.erb