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.
Yes we should still have an example of installing a specific version since I also tore out the installer filename section. I'll add something...