docs/concept: Best practices for schemas

[jpluscplusm]

A guide which expresses the project's current suggestions on defining schemas.

This might include:

• only use required or optional field constraints
  • prefer optional field constraints (as they can be "overridden" with a required field constraint elsewhere, but not vice versa)
  • no regular fields
• don't specify defaults (as they prevent schema consumers from specifying defaults)
• don't mix policy and schema (TODO: expand "policy vs schema" here/elsewhere)
• ...