docs: rust: point directly to the standalone installers

docs: rust: point directly to the standalone installers

The Quick Start guide points to the Rust programming language front
page when it mentions the possibility of using the standalone
installers instead of `rustup`.

This was done to have a hopefully stable link, but it is not too
helpful: readers need to figure out how to reach the standalone
installers from there.

Thus point directly to the page (and anchor) with the table that
contains the standalone installers (plus signing key etc.).

If the link breaks in the future, we can always update it as
needed. And anyway having the full link includes the domain and
gives more information about where the old docs were in such
a broken link case, which may help.

Link: https://lore.kernel.org/linux-doc/CANiq72=gpzQyh1ExGbBWWNdgH-mTATdG5F600jKD1=NLLCn7wg@mail.gmail.com/
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Link: https://lore.kernel.org/r/20230306220959.240235-1-ojeda@kernel.org
[ Removed "install ``rustup``" ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>