Clarify getting started instructions for git checkouts

Fixed best practice

The default branch is already devel (as of June 14, 2012), so there's
no need to check this one out explicitly.  Also, it was being checked
out before we had even entered the source folder, which was obviously
a mistake.

simple docs request about using ${var}

Add a note about how it is possible to use ${var} in addition to $var

Found a couple more broken doc links

removed '-' from ansible calls

Fix misspelling of necessary

I noticed a few small issues in the new docs

Remove outdated note about apt module

Undocument override hosts (gone in 0.4, use -i "host1,host2" if you must, but want to leave that undocumented
as it's not a best practice so much.

Clarifications/fixes for executable inventory examples