Document client extensions

[andrew-walford-prisma]

We need to document the client extensions feature.

This will not be released in 4.5.0, but work is underway.

Pierre's proposal is here: https://github.com/prisma/prisma/issues/15074

Client team doc: https://www.notion.so/prismaio/Client-Extensions-46b162f89e204e7e9b5b4e241da30e86

Design doc (includes examples we could use in the docs): https://www.notion.so/prismaio/Product-Design-for-Client-Extensions-ac2f334ac0a7483e9bb65d1656023f5f

Board for the engineering work: https://github.com/orgs/prisma/projects/74