Document Redis memory space analysis with cupcake-rdb

During a recent analysis we forked and extended a Redis RDB dump analysis tool and published it under cupcake-rdb.

The extensions enable two new types of analysis:

• #2538 (comment 1593195770) - Space distribution flamegraph
• #2538 (comment 1593756454) - Differential flamegraph

We should document these new tools. A few places that could use with improved docs:

• cupcake-rdb README
• redis runbook (could also be its own document in the redis directory)