[Docs] Backport docs to 7.0

[bmorelli25]

Backports the following commits to 7.0:

• [Docs] 7.0 Breaking changes, Release highlights, Migration assistant, ECS changes ([Docs] 7.0 Breaking changes, Release highlights, Migration assistant, ECS changes #1940)
• [docs] Add apm_user role ([docs] Add apm_user role #1916)
• docs: add opentracing bridge ([docs] Add OpenTracing bridge #1887)
• [docs] make update-beats-docs ([docs] make update-beats-docs #1958)

[bmorelli25]

Thanks for catching those @simitt! I've edited the commits. I'll merge once I can get #1958 into this backport as well.