Nested Markdown lists get flattened in generated HTML

Nested Markdown lists result in incorrect HTML generated, which puts them all at the same level, unless there is a switch between ordered and unordered lists, in which case they're properly nested. Repro:

```common-lisp
(defsection @repro (:title "Incorrect HTML generated from Markdown")
"Consider this list:

- Item 1
- Item 2
- Subitem 1
- Subitem 2
- Item 3
* Subitem 3
+ Sub-subitem 1
1. Sub-sub-subitem 1
2. Sub-sub-subitem 2")
```
```
CL-USER> (40ants-doc/builder:update-asdf-system-docs clutch/docs::@repro :clutch)
[18:44:10] 40ants-doc/highlight highlight.lisp (download-highlight-js) -
METADATA file lists same languages and theme. Skipping download of Highlight.js
#P"/home/mathrick/Dev/clutch/docs/"
#P"/home/mathrick/Dev/clutch/docs/repro/index.html"
```
This results in the following output: