[Docs] Backport to 6.x: multiple doc changes (elastic#9319)

* Add linux command examples (elastic#9189)

* Update command reference with more info about Functionbeat (elastic#9203)

* [Docs] Add info about accessing logs in serverless environments (elastic#9241)

docs/index.asciidoc

@@ -14,6 +14,7 @@ include::{asciidoc-dir}/../../shared/attributes.asciidoc[]
 :libbeat-docs: Beats Platform Reference
 :deb_os:
 :rpm_os:
+:linux_os:
 :no_dashboards:
 
 include::../../libbeat/docs/shared-beats-attributes.asciidoc[]