Rust changes for v6.7

A small one compared to the previous one in terms of features. In terms
of lines, as usual, the 'alloc' version upgrade accounts for most of them.

Toolchain and infrastructure:

 - Upgrade to Rust 1.73.0.

   This time around, due to how the kernel and Rust schedules have
   aligned, there are two upgrades in fact. They contain the fixes for
   a few issues we reported to the Rust project.

   In addition, a few cleanups indicated by the upgraded compiler
   or possible thanks to it. For instance, the compiler now detects
   redundant explicit links.

 - A couple changes to the Rust 'Makefile' so that it can be used with
   toybox tools, allowing Rust to be used in the Android kernel build.

x86:

 - Enable IBT if enabled in C.

Documentation:

 - Add "The Rust experiment" section to the Rust index page.

MAINTAINERS

 - Add Maintainer Entry Profile field ('P:').

 - Update our 'W:' field to point to the webpage we have been building
   this year.
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