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. 

In the array section above, you explicitly mentions `If you specify the same
field multiple times, it appends to the array.`

Is it the same for hash or not allowed...?