doc: Update our tier support
This modifies our listing of tiered platforms a few ways:
* All lists are alphabetized based on target now
* Lots of targets are moved up to "Tier 2" as we're gating on all these builds
and official releases are provided (and installable via rustup).
* A few targets now list having a compiler + cargo now as well.
No more platforms have been moved up to Tier 1 at this time, however. The only
real candidate is ``x86_64-unknown-linux-musl`, but that's not *quite* to a tier
1 level of quality just yet so let's hold off for another release or so to iron
it out a bit.