Refactor: Manage site content in Markdown with layouts

This is a non-trivial, behind-the-scenes rebuild of how my site comes together. Instead of being a Jekyll site that serves static HTML, it is now a Jekyll site that builds from modular content written in Markdown, which is then generated into static HTML. Virtually nothing changes in how the site actually appears, but this is almost a total rewrite and leverages the more powerful functionality that Jekyll, Front Matter, and Liquid syntax offers.

This is a really big "Feels Good" moment, because I have wanted to do something like this for my site for years! Before, I didn't have the technical knowledge to really understand how to do it. But now, I was able to do this in a few hours of work. A little bit of a confidence booster for me. 😄

This will warrant a new git tag.