adding converted static asciidocs

This PR contains the static documentation currently hosted on logstash.net, manually converted from markdown to asciidoc format. Still need to decide on overall book structure, which will include both these docs plus the generated plugin docs via @electrical. 

This line is difficult to explain to new users as it concerns only deprecated config (type, tags, exclude_tags).
As filtering must be done with conditionals only, this should be removed asap to avoid confusion.