Support equation rendering

Hugo docs here https://gohugo.io/content-management/formats/ lead to this: https://github.com/oblitum/hugo-theme-slim/commit/2726427d5899720447d90177824ab26996bb0750, which works with minimal effort, IF `mmark` content is used instead of blackfriday markdown. `mmark` is a default supported format, and can be used by simply changing the file extension, or setting `markup = "mmark"` in front matter.