refactor(embassy::net): Enable building without actual driver

[chrysn]

Description

The net module used to not get linted and documented because its code depends on some driver to be present.

This adds an uninhabited dummy driver, which gets selected unless anything else is.

Issues/PRs references

#493 will create a module that should be documented and linted but that depends on net.

There was some prior discussion of this in the riot-rs channel. The alternative to the dummy approach was to gate the #[cfg(feature="net")] part of riot-rs-embassy -- but that would hide that code from the linter, and the linter promptly did find something to enhance.

Change checklist

• I have cleaned up my commit history and squashed fixup commits.
• I have followed the Coding Conventions.
• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.

Fixup commits are initally left in because @ROMemories already saw the first of these commits.

[ROMemories]

Couple of nits and we're good!

I'll add the missing doc comments on the network items in a follow-up PR.

[chrysn]

Thanks, all in and squashed.