Surface Invoke's utility as a replacement for subprocess better in the docs

[bitprophet]

Realized that while it's trivial to do, the docs don't actually make it obvious that you can, and IMHO should (in lieu of subprocess and friends):

from invoke import run

result = run("my shell command here")

Instead, the docs only really show you this in the tutorial, and that only within the context of, well, a Context. There's no explicit mention of "psst this one thing that has a lot of dev time behind it and is very powerful, can be used a la carte".

Relatedly, the README/index doesn't link anywhere else in the docs, which seems...bad. Maybe fix that at the same time.