Document use of jinja-markdown

[scottyhq]

I'm trying to refresh my memory of why we use jinja-markdown and have very specific pins of versions for it. It seems unmaintained but also very simple. Perhaps there is an alternative now that a few years have passed... https://github.com/jpsca/jinja-markdown

# Dependencies of jinja-markdown:
  - jinja2>=2.11
  - pygments>=2.6.1
  - pymdown-extensions>=7.1
  - pip:
    - jinja-markdown==1.210911

cc @lsetiawan @jomey