Use CommonMark to Improve Performance and to Avoid Deprecation

[gkapfham]

The site is currently using rdiscount, which is known to be slow and will be deprecated in a future version of Jekyll. I need to upgrade to a newer version! For details about the performance improvements, refer to the following pages:

• https://boris.schapira.dev/2018/11/jekyll-build-optimization/
• https://forestry.io/blog/how-i-reduced-my-jekyll-build-time-by-61/?q=&hPP=10&idx=blog&p=0

With that said, it is important to note that rdiscount is "permissive" in that it will repair some of the latent mistakes in my Markdown and HTML. The CommonMark parser, in contrasts, will point out these problems and render incorrect HTML source code. I think that it would be useful to fix these defects, but that will take extra time and not lead to a new feature.