Merge branch 'add_ha_diagrams' into 'master'

Add new HA diagrams [ci skip]

## What does this MR do?

Adds new HA diagrams created by @lukebabb (Thanks @lukebabb!)

See merge request !4289

doc/administration/high_availability/README.md

@@ -19,6 +19,8 @@ Components/Servers Required:
 
 - 2 servers/virtual machines (one active/one passive)
 
+![Active/Passive HA Diagram](../img/high_availability/active-passive-diagram.png)
+
 ### Active/Active
 
 This architecture scales easily because all application servers handle
@@ -26,6 +28,8 @@ user requests simultaneously. The database, Redis, and GitLab application are
 all deployed on separate servers. The configuration is **only** highly-available
 if the database, Redis and storage are also configured as such.
 
+![Active/Active HA Diagram](../img/high_availability/active-active-diagram.png)
+
 **Steps to configure active/active:**
 
 1. [Configure the database](database.md)