Document the scaling approach for each service

[annanay25]

The Operations section in the docs needs a snippet on the recommended scaling approach for each service and any gotchas associated.

For instance - can we scale up ingesters rapidly - impact on read/write path, can we scale up query frontends beyond 2 - impact on read path?

[knylander-grafana]

This might be associated with the capacity planning. See #543

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