Document build from release archive and fix ToC link
Document build from release archive and fix ToC link

Update the documentation to include minimal instructions for building
the velero binary from the GitHub Release "Source code" archive.

Correct the Download link in the Table of Contents for
build-from-scratch.md.

Signed-off-by: Darren Hart <38f9eaa30910c40b06f5a60020b1222f2a953801@vmware.com>

heptio/ark,heptio/ark