tvix: document using tvix crates outside of depot

Opened by flokli at 2024-12-27T18·24+00

There's a few things to be aware of when using certain tvix crates (other than nix-compat):

PROTO_ROOT needs to be set to a checkout of the depot (or something containing only the .proto files in the same structure
TVIX_BUILD_SANDBOX_SHELL needs to be set to something (only for tvix-build)
protoc needs to be in $PATH (or PROTOC set)

This should be documented somewhere in tvix/docs, propaply also adding some example code that shows how the parts in Cargo.toml should look like.