[docs] Add APM to simplified security docs

[bmorelli25]

• Persists the APM xpack security changes in [docs] xpack security improvements from Beats apm-server#2228. Waiting for approval on the APM side before marking this as ready for review.
• Follow up to DeDe's work on Simplify Beats security docs #11329.

[dedemorton]

A couple of minor changes. I didn't build to test the output, but LGTM from eyeballing it.

[bmorelli25]

In testing I found a build error with heartbeat: they use the no_dashboards attribute which was causing asciidoctor to attempt to build a blank "Privileges needed by {kib} users" table (it doesn't like that). Updated the attribute in 72f854b to only rule out APM Server.