Update jekyll to use a theme gem

[0xdevalias]

Currently we use a legacy 'inline' theme, rather than a gem based theme like Jekyll has supported for a while now:

The goal of gem-based themes is to allow you to get all the benefits of a robust, continually updated theme without having all the theme’s files getting in your way and over-complicating what might be your primary focus: creating content.

There are many options out there, or we could even make our own:

• http://jekyllthemes.org/
• https://jekyllthemes.io/
• https://jekyll-themes.com/
• https://jamstackthemes.dev/ssg/jekyll/
• https://jekyllrb.com/docs/themes/#creating-a-gem-based-theme
• https://jekyllrb.com/docs/themes/#publishing-your-theme