Improve Heat Map documentation

[bud1979]

In creating a heat map, We need more documentation around how to create the visualization. There needs to be mentions of having both row and column information filled out, and descriptions on what types of data fit best into a heat map.

Attached is an example of web log response codes, for an example of what could be done

Context

Documentation needs to be updated to better inform the end user on how to create a heat map. Make the documentation easier to use.

Expected Content

Overall descriptoin of how to use the aggregation of "Heat Metric"

Your Environment

• Graylog Version: 3.0.2
• Elasticsearch Version: 6.8
• MongoDB Version: 4