Document code intel resource requirements for postgres DB and codeintel-db

Created by: varungandhi-src

Recently, we have run into issues where a customer reported precise code intelligence disappearing unexpectedly. This is believed to have come about because:

• The memory limit for postgres was fairly low.
• Uploading LSIF for a large monorepo resulted in a large amount of data for commit graph visibility being inserted into the main postgres DB, causing an OOM.

The resource estimator at https://docs.sourcegraph.com/admin/install/resource_estimator currently doesn't have information on how much resources are recommended for the postgres DB. Similarly, it is lacking information on how much resources are appropriate for codeintel-db (which is a separate deployment).

It would be valuable to add that information to the resource estimator for smoother on-boarding.