Beats Doc Updates for 5.2

[dedemorton]

This is a meta issue to track Beats documentation work for 5.2.

If you have trouble clicking an item in the checklist to mark it as complete, open this comment in edit mode and change the source to [x]. Seems to be a bug in ticking the check boxes in this issue.

5.2:

• [DeDe] Add Heartbeat docs. See Add docs for heartbeat #3247
• [DeDe] Test Getting Started section of Heartbeat docs.
• Add/edit docs for system.socket in Metricbeat. Experimental feature only on Linux.
• Add/edit docs for Prometheus module in Metricbeat. See Add edits for prometheus module docs #3520
• Edit HAProxy docs. See Edit haproxy module docs #3216.
• Document new config options for processors. See Document new config options for decode_json_fields processor #3443

NOTE: Moved wish list and future items to 5.3 meta issue here: #3539