book claims nightly rust is still needed

[rursprung]

the book still claims that a nightly rust version is needed which isn't the case anymore:

embassy/docs/pages/new_project.adoc

Line 135 in 015e3ae

Before we can build our project, we need to add an additional file to tell cargo to use the nightly toolchain. Copy the `rust-toolchain.toml` from the embassy repo to ours, and trim the list of targets down to only the target triple relevent for our project — in this case, `thumbv7em-none-eabi`:

the docs also uses nightly (either as features or for rustc) in a couple of other places - i'm not sure which of these should be adapted and which not (i was hoping that at least the whole "for beginners" section is nightly-free)