Create definitions list, update module 'type' entries to link back to def list entry

[deoren]

While reviewing some doc work, I learned (https://github.com/rsyslog/rsyslog-doc/pull/593#issuecomment-369497032) that the parameter types that I thought was only specified by convention are actually handled as very specific (enforced) types.

To make it clear to anyone reading the docs, I think we should either update the 'types' column to link back to entries in a definitions doc (yet to be written) or have each type entry be a separate link to the specific explanation for that type (e.g., "word" links back to the "word" type description, "string" links back to string, etc).