MAINTAINERS: update Rust webpage

A few months ago we started a webpage for the Rust subsystem and
the overall Rust for Linux project [1].

The current `W:` field of the Rust entry points to the GitHub
repository, since originally we kept information in a `README.md`
file that got rendered by GitHub when visiting that URL.

That information was moved into the webpage and got expanded.
The webpage is also nowadays the entry point to the project,
and we pointed the "Website" GitHub metadata field to it.

Thus update the `W:` field to point to the actual webpage.

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

MAINTAINERS

@@ -18709,7 +18709,7 @@ R:	Andreas Hindborg <a.hindborg@samsung.com>
 R:	Alice Ryhl <aliceryhl@google.com>
 L:	rust-for-linux@vger.kernel.org
 S:	Supported
-W:	https://github.com/Rust-for-Linux/linux
+W:	https://rust-for-linux.com
 B:	https://github.com/Rust-for-Linux/linux/issues
 C:	zulip://rust-for-linux.zulipchat.com
 T:	git https://github.com/Rust-for-Linux/linux.git rust-next