Best Practices document

https://www.graphile.org/postgraphile/best-practices/

- Don't expose more than you need to
- Adding indexes (`--no-ignore-index`)
- Adding permissions
- Creating roles
- Using namespaces
- Using `@omit`

Start by reviewing the existing page at https://www.graphile.org/postgraphile/best-practices/ and compare it with the listed topics. Update the Best Practices document to cover limiting exposure, indexes, permissions, roles, namespaces, and @omit; done means these topics are documented on the page.

Written by the indexing model from the issue text.