[Docs] Add sizing recommendation for backend pod

[wilfriedroset]

Describe the solution you'd like
The documentation about "size the cluster" could add support for the new SSD chart with the three part deployment. The current version as of now only show write/read.
Example:

loki:
  authenabled: false
read:
  replicas: 75
  resources:
    requests:
      cpu: 3
      memory: 6
    limits:
      cpu: 3
      memory: 8
write:
  replicas: 58
  resources:
    requests:
      cpu: 1
      memory: 6
    limits:
      cpu: 2
      memory: 12

Describe alternatives you've considered
N/A

Additional context
https://grafana.com/docs/loki/next/installation/sizing/

[wilfriedroset]

Please also note that unit must be specified for the memory. Without any, the default will be taken into account. The default unit for memory is byte and we don't want to start with that few memory.

[LeszekBlazewski]

This would be really helpful!

The docs also don't mention any sizing recommendations.