Update tutorial with mirage-skeleton#361 changes

[reynir]

This is a companion PR for https://github.com/mirage/mirage-skeleton/pull/361. The example is simplified, and the example no longer includes ~deps or custom dune rules. Instead, the example now explains Mirage.if_impl and differences in how block devices are specified in the hvt and unix targets.

[reynir]

The PR has been updated with changes in https://github.com/mirage/mirage-skeleton/pull/367. The noop example has changed in mirage-skeleton and the tutorial has not been updated, and in https://github.com/mirage/mirage-skeleton/pull/367 I simplify the noop example so we can still delay explaining functors.

[samoht]

Looks great!

[samoht]

Do you mind rebasing this on master to fix the build issues? Thanks!

[reynir]

Done :-)

[samoht]

Thanks!