Decide whether to keep or update the old "High Availability" doc

[desimone]

Follow-up to #1726

What's incorrect or missing?

• HA details partially merged with config docs
• Some HA concepts may be outdated

What's the resolution?

• Either remove the old doc or fully rewrite as "Scalability & HA"
• Confirm no critical info is lost in the merge

\cc @kenjenkins i know this was particularly of interest to you