Add Redis memory debugging docs
Add Redis memory debugging docs

We recently went through an incident involving Redis running
out of memory. The two immediate solutions were to:
1. Increase the size of the Redis nodes, to buy time/memory
   while we investigated the root cause
2. Find the key using large amounts of memory (maps to a
   an app's queue), and fix the underlying app bug.

I have update the doc to reflect the various steps we took
for future incidents.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs