Add on-prem stack install demo (#2597)

* Add on-prem stack install demo

* Add steps 3 & 4

* Add Kibana install steps

* Add steps for Fleet Server

* Add agent & view data steps

* I can't count, apparently.

* fixup

* fixup

* Fix agent and kibana steps, including port 8220 fixes

* Add certificates section

* typo

* Split security steps into a 'part 2'  guide

* Cleanup

* Improve note about jumping to second guide

* Fix start and stop; fix chown

* Add Kibana SSL steps

* Fleet steps

* Finish Fleet & Agent steps

* Fix intro

* Cleanup

* Add overview diagram

* Add conclusion

* Add warning not to start the service yet

* re-enable test flags

* Address Eric's comments

* Fixup

* Fixup

* fixup

* Fixup

* Fixup

* fixup

* fixup

* Fixup; and unset version variables

* Latest review comments

* Use full URLs for package list

(cherry picked from commit da51380)

Author: kilfoyle

docs/en/install-upgrade/images/install-stack-metrics-dashboard.png

@@ -17,6 +17,10 @@ include::overview.asciidoc[]
 
 include::installing-stack.asciidoc[]
 
+include::installing-stack-demo-self.asciidoc[]
+
+include::installing-stack-demo-secure.asciidoc[]
+
 include::air-gapped-install.asciidoc[]
 
 include::upgrading-stack.asciidoc[]