improve "custom build binaries key" docs

[ashleygwilliams]

# A list of all binaries your package will build and install
binaries = ["quickstart-example"]

it's not clear to me what goes here- is it the name, do i include an extension?

[ashleygwilliams]

we should probably link to the config docs (or ideally include the information that is in the config docs https://opensource.axo.dev/cargo-dist/book/reference/config.html#packagebinaries)

[Gankra]

Currently assuming this is a docs bug for the "everyone else" quickstart.