Add sizing guidance to docs

[jeffreyscarpenter]

We are frequently asked for information on sizing Stargate deployments, the need becomes even greater for Stargate v2 with separate API services. We should have a documentation page that provides recommended sizing guidance

• minimum memory and CPU for the coordinator and each API service
• how to determine/tune number of Stargate nodes vs. Cassandra cluster size
• how to determine how many replicas of each API service