Add documentation for Cortex-M23/M33/M35P

[NZSmartie]

This is sort of a cross-posted issue from https://github.com/rust-embedded/cortex-m-quickstart/issues/94

I'm getting started with a ATSAML10xxxx series project that uses Cortex M23 (ARM v8 baseline)

Since I'm new to Rust development (but already an experienced embedded developer), I've been following The Embedded Rust Book - Installing the tools. Specifically it suggests adding the following targets with rustup target add

• thumbv6m-none-eabi
• thumbv7m-none-eabi
• thumbv7em-none-eabi
• thumbv7em-none-eabihf

As for ARM v8 baseline, mainline and mainline with extension, the targets are respectively:

• thumbv8m.base-none-eabi (e.g. Cortex M23)
• thumbv8m.main-none-eabi (Not sure which arch uses this instruction set specifically 🤷‍♀️)
• thumbv8m.main-none-eabihf (e.g. Cortex M33, CortexM35P)

Could these be added to src/intro/install.md and also other places I've yet to come across?

https://github.com/rust-embedded/book/blob/0cd2ca116274b915924c3a7e07c1e046b6f19b77/src/intro/install.md#L14-L40

Maybe a reference to the Cortex-M comparison table might help with picking out the right instruction set

[eldruin]

Sounds fine to me, although that list is getting longer. Could you send a PR to this repo adding these?