download: explain that packages are for 64-bit arch only

Thanks to Sriram Narayanan for pointing this out.
Merge pull request #1 from mgeisler/master

download: explain that packages are for 64-bit arch only
Fixes front page typo
Merge pull request #2 from samuliheljo/master

Fixes front page typo
Fix broken links
Merge pull request #4 from willkelly/master

Fix broken links
typo fix
update links to ReadTheDocs pages

URLs were 404-ing because docs are located at /en/latest/* these days.
typo fix, as well as noting that Zebra is beta
added link to zerovm blog from menu on each page