Actually, it’s not. The “Filtered HTML” format that is in your screenshot is from a default Drupal installation. We don’t use that. We provide our own, and just call it “Default”. This way, we simplified things by taking an opinionated approach in farmOS and said “we’ll only have one text format”.
So, I’m proposing just enabling Markdown parsing on our existing “Default” text format - not adding a separate one alongside it. Thus, from a user perspective it will all work the same as before, except now Markdown formatting will be parsed into HTML automatically. No need to select a different format from a dropdown (or understand what any of that means).
@rmattb Giving this some more thought… with Markdown you don’t use <ol> and <li> tags directly, so for numbered lists you have to type out the numbers you want anyway, which means you can achieve the “hierarchical/outline/legal numbering” you’re describing… it just requires manually typing them. 
For example (this forum uses Markdown too):
- First item
1.1. First sub item
1.2. Second sub item - Second item
- Third item
Adding additional CSS, like you describe, is only necessary if you want to write your lists in HTML (using <li>) tags. But with Markdown you don’t need to do that. So maybe we don’t need anything else?