Add documentation ARM v8 Baseline and Mainline (Cortex-M23/M33/M35P)

[NZSmartie]

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 README.md and also other places I've yet to come across?

cortex-m-quickstart/README.md

Lines 17 to 23 in ae6ad14

	- `rust-std` components (pre-compiled `core` crate) for the ARM Cortex-M
	targets. Run:
	``` console
	$ rustup target add thumbv6m-none-eabi thumbv7m-none-eabi thumbv7em-none-eabi thumbv7em-none-eabihf
	```

and

cortex-m-quickstart/README.md

Lines 77 to 84 in ae6ad14

	``` toml
	[build]
	# Pick ONE of these compilation targets
	# target = "thumbv6m-none-eabi" # Cortex-M0 and Cortex-M0+
	# target = "thumbv7m-none-eabi" # Cortex-M3
	# target = "thumbv7em-none-eabi" # Cortex-M4 and Cortex-M7 (no FPU)
	target = "thumbv7em-none-eabihf" # Cortex-M4F and Cortex-M7F (with FPU)
	```

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