Improve mkdocs toc

[williamdemeo]

Description

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• Any semantic changes to the specifications are documented in CHANGELOG.md
• Code is formatted according to CONTRIBUTING.md
• Self-reviewed the diff

[carlostome]

Looks good! Just a couple of comments:

• In the home page "Home" on the left column looks weird.
• The symbol > to expand a directory looks a bit odd all the way to the right.
• I'd hoist Core to its own tab.

[williamdemeo]

Looks good! Just a couple of comments:

• In the home page "Home" on the left column looks weird.
• The symbol > to expand a directory looks a bit odd all the way to the right.

Not really sure what we can do about these two items... but I'll try to figure something out.

• I'd hoist Core to its own tab.

Okay, that's how it was before. I'll revert. 👍🏼

Also, I don't really like that Interface is a mix of two very different kinds of modules:

1. STS and ComputationalRelation
2. Type class definitions and instances

I suggest we leave STS and ComputationalRelation as "top-level" submodules of Interface and move the type class modules into a subdirectory called TypeClasses. @carlostome what do you think? I'll effect and push that change now and wait for the CI to pass before merging. I'll also try to fix the location of the > symbols that expand directories.

[williamdemeo]

As for the "Home" on the left side of the index page, whatever we name the tab at the top will show up on the far left. I'm not sure if we can do anything about that. Let's leave it for now.

As for the >, I'm not sure if we can avoid the right-alignment, but I did manage to make it a little less far to the right by adjusting .md-sidebar--primary .md-nav__list at the bottom of custom.css.