Improve binaryTargets documentation

[janpio]

binaryTargets are a way to tell the Prisma Client (prisma-client-js) generator for which platforms (operating systems) it should download and store engine files. The current documentation is a good start, but not great as it is missing some bits and hard to understand for users.

Current documentation:

• Mentioned here: https://www.prisma.io/docs/reference/api-reference/prisma-schema-reference#fields-1
• Options listed: https://www.prisma.io/docs/reference/api-reference/prisma-schema-reference#binarytargets-options
• Kinda explained here: https://www.prisma.io/docs/concepts/components/prisma-schema/generators#binary-targets-prisma-client-js
• Deployment guides use it: https://www.prisma.io/docs/guides/deployment/deployment-guides/deploying-to-aws-lambda#binary-targets-in-schemaprisma

Problems:

• No real explanation what setting a value actually does and when it should be used: It downloads additional engines to the ones it would download by default, for the current platform. It is useful when you build on one platform, but deploy to another platform (like e.g. build locally on Mac, but then deploy that build via a Zip file to AWS Lambda, or you build with netlify which runs on Debian but that then deploys to AWS Lambda, which runs on Redhat Linux). It is not necessary if you have a build step on your deployment platform itself as this will already download the correct binary.
• Wrong explanation and connection made, like e.g. https://www.prisma.io/docs/concepts/components/prisma-schema/generators#custom-binary-targets (this has nothing to do with "custom binary targets", but of course has a similar goal in using a different than default binary)
• Deployment guides can not and do not link to an explanation of what it is
• The binaryTargets property is technically only a property of prisma-client-js, not all generators so is more or less documented in the wrong place in the schema. The list of supported platforms should probably be moved somewhere else more for the Client vs. at the current location with schema
• Related content like https://www.prisma.io/docs/reference/api-reference/environment-variables-reference#prisma_cli_binary_targets, which is how you can define similar things for CLI, is not crosslinked