xtask refactoring

[MabezDev]

This is somewhat of a stub issue which we'll likely want to expand and split up.

The xtask has grown organically over the last year and we should refactor various parts of it:

• Not enough logging, it's hard to tell sometimes where a failure happened
• Toolchain selection is all over the place
• Multiple ways to run tests/examples
• Inconsistent argument flags to some sub commands
• Currently build-documentation-index requires build-documentation to have been run prior; this makes sense technically, but may trip some people up.
• Serving the docs locally with a server, something like xtask build-documentation --server or something (Xtask/latest redirect #3231)
• Documenting the documentation process would probably not be a bad idea either. This can just be added to xtask/README.md or somthing.