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.
Maybe just leaving in the `-s 1.0.1` section here would have been better, we're used to bumping the version in this doc anyway and upon reflection I like being explicit.