Clean up schema spec

[janpio]

This PR tries to bring the Schema spec to a state that is closer to what Prisma will launch with.

The following changes were made:

• Remove mongodb as supported provider
• Remove generator block namespaces (https://github.com/prisma/specs/pull/424)
• Model Block: Remove information on backing (https://github.com/prisma/specs/pull/423)
• Core Data Type to Connector: Remove unsupported Connectors (https://github.com/prisma/specs/pull/422)
• Referencing fields that are not @id: Add detail about references
• Remove onDelete (https://github.com/prisma/specs/pull/421)
• Remove Type Specifications (https://github.com/prisma/specs/pull/420)
• Remove Type Definition (https://github.com/prisma/specs/pull/420)
• Remove Embed Block (https://github.com/prisma/specs/pull/419)
• Remove Env default (https://github.com/prisma/specs/pull/418)
• Env Function: Remove CLI spec detail
• Remove Importing schemas (https://github.com/prisma/specs/pull/417)

All paragraph/headline removals were turned into Pull Requests that add the content again (see in (...) above).

---

Future changes, that were not part of this PR:

• Move Auto Formatting to its own file
• Check voice (no "we" and "our" etc)
• Some of the examples for "attributes" etc still use stuff that got removed in this PR