Logstash 5.3 doc fixes

[dedemorton]

This list is a work-in-progress. Need to meet with dev to prioritize/refine goals

5.3:

• Document Filebeat modules (see Logstash docs and configs for Filebeat modules #6542)
• Generate plugin docs for 5.3. Make sure the manual tweaks made in Update plugin docs to cleanup formatting issues logstash-docs#353 get in. Make sure the docs for new plugins that got merged get generated.
• Document content-type header integration compatibility (See Documenting content-type header integration compatibility #6718)
• Add "at least once" delivery guarantees to the PQ docs.
• Make remaining plugin changes (I'll move items from here to this list after the dust settles).
• Bug fixes and enhancements tagged for 5:3, including (but not limited to):

Moved "backlog" to #6850.