MAINTAINERS: add Maintainer Entry Profile field for Rust

The `P:` field allows a URI, and we have been adding information
on how to contribute to the Rust subsystem to our webpage for a
while now (and other relevant topics, e.g. branches, third-party
crates, Rust unstable features...).

The webpage/document does not currently follow the headings of
the Maintainer Entry Profile, but it does nevertheless answer
some of the questions we have got so far from contributors and
it covers partially some of the topics there.

Thus link to our "Contributing" document [1].

Link: https://rust-for-linux.com/contributing [1]
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20230830165949.127475-2-ojeda@kernel.orgSigned-off-by: Miguel Ojeda <ojeda@kernel.org>

MAINTAINERS

@@ -18712,6 +18712,7 @@ S:	Supported
 W:	https://rust-for-linux.com
 B:	https://github.com/Rust-for-Linux/linux/issues
 C:	zulip://rust-for-linux.zulipchat.com
+P:	https://rust-for-linux.com/contributing
 T:	git https://github.com/Rust-for-Linux/linux.git rust-next
 F:	Documentation/rust/
 F:	rust/