docs: add a page for performance best practices

add a section in docs with best practices and tips to improve performance

a few sample points:

- analyzing queries to figure out bottlenecks
- setting up db indexes
- combining multiple requests into one if possible
- recommended DB connections configuration
- optimising subscriptions

[Useful resoruce](https://github.com/hgiasac/blog-builder/blob/master/pages/posts/2020-05-15-Tuning-Hasura-at-Scale.md).

Inspect the existing documentation structure first, then read the linked Hasura performance resource for context. Create a performance best-practices section covering query bottlenecks, database indexes and connections, request consolidation, and subscription optimization; done means the guidance is clearly organized in the appropriate docs location.

Written by the indexing model from the issue text.