docs(install-deisctl): remove extra installer links and simplify doc

This shortens the instructions for installing `deisctl`, removes redundant links to the installers, excises details of the installer filename patterns, and adds a note about running `deisctl refresh-units` if you're building it from source. Overall I think this reads better.
Should we still say how to install specific version? Some users won't necessarily always be upgrading to latest, and may need (what is to us) an older deisctl.