Add section with alloc only but not libstd

When https://github.com/rust-lang/rfcs/pull/2480 is stabilized and implemented, it should be possible to create binaries that don't depend on `std` but do depend on `alloc`. This would be a step between `no_std` and `no_main`, because at least you could use the heap. The actual sample hello world code would probably not change much. It would probably be best to show off using a `Vec` in this instance.

This could probably be added now as a `nightly` subproject.

Start by reviewing the existing no_std and no_main documentation and the linked RFC 2480, confirming whether the alloc-only setup is stabilized and implemented. Add an alloc-only section with a Vec-based hello-world example when the prerequisite is available; the documentation should clearly distinguish it from no_std and no_main.

Written by the indexing model from the issue text.