Update to Jekyll 4, add docs for Front Matter layout defaults

[joeleenk]

We want all of our open source projects to be able to use this theme with as minimal setup and repetition as possible. But it appears we need Jekyll 4 to inherit site variables from the theme config. Inheriting the variables also seems to only work from GitHub hosted Gems, and not from RubyGems, so we'll switch to using the Gem from this repo. (If someone knows why or some other way to solve this, please let me know.)

This PR also:

• Adds docs for adding Front Matter layout defaults to the config file so that all markdown files don't need to specify a layout. This fixes a problem we found with Webpack Helpers not loading the theme properly if we removed the github-pages gem. Note: this does not appear to inherit and will need to be set in the site's config file.
• Removes the HM homepage link from the header, so that it's not forced to inherit to all of the other sites. (If those sites also want links in the header, it gets a bit crowded.)

[kadamwhite]

Great solution!