rust: add extensions to the pin-init crate and move relevant documentation there

In preparation of splitting off the pin-init crate from the kernel
crate, move all kernel-specific documentation from pin-init back into
the kernel crate.

Also include an example from the user-space version [1] adapted to the
kernel.

The new `init.rs` file will also be populated by kernel-specific
extensions to the pin-init crate by the next commits.

Link: https://github.com/Rust-for-Linux/pin-init/blob/c1417c64c71229f0fd444d75e88f33e3c547c829/src/lib.rs#L161

 [1]
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250308110339.2997091-4-benno.lossin@proton.me


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>