style: subordinate TOCs

[medubelko]

Situation

In an upcoming change to the standard docs IA, we will require a separate TOC section:

Request

1. Modify Furo's TOC scaffolding so we can use custom classes in the toctree directive:

   .. toctree:
       :class: toc-sub

2. Add sub-TOC styling to the default CSS:

   .toc-sub {
     margin-top: 1em;
     padding-top: 1em;
     border-top: 1px solid var(--color-background-border);
   }