Massive update to the `download` page

A lot of the instructions are old and out of date. Technically, they
still work, but they're not super relevant for the current state of the
project and the documentation on zerovm.readthedocs.org.
Merge pull request #12 from larsbutler/update-download-page

Massive update to the `download` page
download: typos and formatting fixes
Merge branch 'update-download-page'