Add release highlights section to the Beats documentation

[dedemorton]

Other products document release highlights. We should do the same for Beats.

[dedemorton]

The structure also needs to be cleaned up. These sections should appear together as they do in the docs for other products:

Release highlights
Breaking changes
Release notes

[dedemorton]

Note that the breaking changes for 7.0 should include 7.0-related changes only (right now it also contains 6.x releases).

[ruflin]

• Change to ECS of Beats and modules
• ILM
• New modules for Metricbeat (NATS, Couchdb, MSSQL, AWS)

[dedemorton]

Closed by #11621