The "Releases" section on Github is a little obscure, but from that link you gave to Sigil's main Github page:
https://github.com/user-none/Sigil
you'll find a series of tabs (immediately beneath "Sigil is a multi-platform EPUB editor") labeled "commits," "branches," "releases," and "contributors." The "releases" one is a link that will take you to the downloads for the various installer packages.