Add ES sizing guide for single node clusters

What does this MR do and why?

Related to #456277 (closed)

Add documentation for shard size and replica count recommendations for single-node clusters

Add docs link to rake task output for info on single-node clusters

MR acceptance checklist

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Screenshots or screen recordings

➜ bundle exec rake gitlab:elastic:estimate_shard_sizes
Using approximate counts to estimate shard counts for data indexed from database. This does not include repository data.
For single-node cluster recommendations, see http://gdk.test:3000/help/integration/advanced_search/elasticsearch#number-of-elasticsearch-shards
The approximate document counts, recommended shard size, and replica size for each index are:
- gitlab-development-issues:
   document count: 669
   recommended shards: 5
   recommended replicas: 1
- gitlab-development-notes:
   document count: 159
   recommended shards: 5
   recommended replicas: 1
- gitlab-development-merge_requests:
   document count: 107
   recommended shards: 5
   recommended replicas: 1
- gitlab-development-epics:
   document count: 40
   recommended shards: 5
   recommended replicas: 1
- gitlab-development-users:
   document count: 61
   recommended shards: 5
   recommended replicas: 1
- gitlab-development-projects:
   document count: 56
   recommended shards: 5
   recommended replicas: 1
Please note that it is possible to index only selected namespaces/projects by using Advanced search indexing restrictions. This estimate does not take into account indexing restrictions.

How to set up and validate locally

1. run the rake task

   bundle exec rake gitlab:elastic:estimate_shard_sizes