Update architectures documentation

[mr-cal]

What needs to get done

The architectures documentation needs to be improved, including:

• default behaviors (e.g. core22 builds for all architectures)
• description of architectural envvars
• description of cli options (--target-arch, --build-for)
• what happens in destructive mode (e.g. does the environment get cleaned?)
• validation rules and limitations

I would like to get ideas and feedback from @rpjday.

Why it needs to get done

Architectures are complex and need thorough documentation.

[rpjday]

Working on it ...

[rpjday]

First thing I would do is confirm the full set of valid architectures; also, is there any command that lists them? The way "snapcraft plugins" lists plugins, etc? Is there any command that produces that list?

Also, regarding those new, improved ARCH-related variables, what is the ETA on those dropping? Trying to figure out what I should say about them in the upcoming SC101 course.

[cmatsuoka]

Consider adding a note to example 5 clarifying that Snapcraft won't cross-compile automatically, or maybe avoid using the term "cross-compilation" altogether.