Explain "Schema Isolation" better

[wolfgangwalther]

To improve the docs, we should heavily extend the Schema Isolation explanation. This could include everything that is related to different schemas and everything on how to work with views the best way - since those are the core component when isolating schemas.

The following issues in postgrest-docs are related to this:

• Return Full object from INSERT Trigger postgrest-docs#17
• Add a hint how to prevent auto-updateability of a view postgrest-docs#366
• Clarify issues of view usage with row level security postgrest-docs#388
• Make the api_owner role first class postgrest-docs#400
• Recommend using views for abstraction, but don't enforce it from the beginning postgrest-docs#531
• Document workaround for custom types in a private schema postgrest-docs#534
• Having more than one internal schema on schema isolation is confusing postgrest-docs#736
• Expand on Schema Isolation postgrest-docs#748