Improve native types docs

[keerlu]

Add a guide explaining:

• how Prisma's type system works
• how to add native type attributes to refine the basic types
• how to use native type attributes when introspecting and migrating

Currently we have some documentation of native types in the connectors and the schema reference, but nothing at top level, and many topics are not covered. Need to discuss the specifics more but would be good to improve this.

See Slack comment here.

Current Notion discussion here.