Docs: Add guide for multiarch snaps

[sed-i]

What needs to get done

We should document how multiarch and snaps work:

• Is there one *.snap file per arch (arm64, amd64, ...), or a single snap can be multiarch?
• For testing purposes, can I cross-build a snap on my machine? E.g. build an arm64 snap on my amd64 machine? What if it's rust/go -- is it then possible to cross build?
• What is the recommended way of setting up CI for multiarch builds? Does it have to involve launchpad?
• What platforms are supported? For example, nushell publishes for x86_64, riscv64gc, armv7, aarch64-pc, aarch64-apple.

Why it needs to get done

Help users understand multiarch snaps so they can produce more compelling snaps.

[dilyn-corner]

Architecture information is... Duplicated, largely, across three different pages: here, here, and here.

I think these pages cover your questions, but the answers should boil down to:

• one .snap per arch, with a potential for an *_all.snap on core22 base snaps
• you can certainly try to cross-build a snap. It isn't impossible, it's just potentially tricky.
• By "multiarch" do you mean a single snap that can run on multiple architectures?
• supported architectures