Update metadata, lots of little style improvements

[cassidyjames]

Kind of a grab-bag of improvements from being forced to use the site a lot today. :) But not too wild. In summary:

• Update GitHub Action to mirror some of the changes upstream and at Fix generation of app files elementary/appcenter-web#96
• Update README with a bit more verbose instructions for building the site
• Simplify the Ruby script to remove a bunch of bits we were never using
• Update default URLs to exclude the trailing slash (makes it easier to copy-paste app IDs…)
• Refresh styles for legibility (larger app titles, contrast protection for app icons)
• Include app summaries on the home page for better search
• Mention using Ctrl+F on the home page to search

Before	After

[cassidyjames]

Going to merge this since it's being used by a partner, plus it's easy to revert if something catastrophic happens.