Add section in docs about how to compile & use binaries

[Jolg42]

Needed: clear steps for people who want to compile the binaries, for example for Alpine or arm (related prisma/prisma#1635)

• We have instructions on how to build in the README here: https://github.com/prisma/prisma-engines#building-prisma-engines

But it’s only "half of it" because this only mention compiling but not how to use the binaries with the env vars

• Best I found about the vars is the spec but there is so much stuff in there I’m afraid it could be confusing https://github.com/prisma/specs/blob/master/binaries/Readme.md#environment-variables

[janpio]

Duplicate of https://github.com/prisma/prisma/issues/1840?

[Jolg42]

kind of, not exactly :)