doc/rustc: describe the uefi target platforms

Add a `platform-support` entry to the rustc-docs for the different
`*-unknown-uefi` targets. This describes in detail how this platform
works, a few basic examples, and how to compile for the platform.

Red Hat is sponsoring my work on this platform, so I am putting myself
down as target maintainer. Co-maintainers are more than welcome to join
me in the effort. Communication is going on off-list to coordinate the
different efforts.

Note that the ultimate goal is to move the UEFI targets to Tier-2 so
bootloaders can be more easily supported in commercial products. This
documentation is the first step towards that goal, but should be a
viable documentation even for the current Tier-3 status of the targets.

I also want to point out that there is an ongoing GSoC-effort to port
the rust standard library to UEFI (by Ayush Singh). While this work is
not necessarily required to get to Tier-2, we definitely should
coordinate the efforts and update the documentation as soon as any such
ports are merged.

Note that the targets are already used by multiple commercial and non
commercial production systems, including, but not limited to:

 * Tianocore-EDK2 (Official UEFI SDK by Intel) comes with rust support
   in its staging repository (not part of any release, yet).
   (https://github.com/tianocore/edk2-staging/tree)
 * Intel's research program "Project Mu" uses the rust UEFI targets to
   show possible future replacements for Tianocore-EDK2.
 * The Rust OS "Redox" uses the UEFI targets for its bootloader.
   (https://www.redox-os.org/)
 * The hugely popular in-depth documentation of OS development in Rust
   by Philipp Oppermann uses the UEFI targets.
   (https://os.phil-opp.com/)

Signed-off-by: David Rheinsberg <david.rheinsberg@gmail.com>

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