Jekyll

This is an umbrella issue that follows up #821 - namely, to re-enable the rules I disabled when upgrading our config.

The core ones are:

• declaration-block-no-redundant-longhand-properties
• no-descending-specificity
• scss/no-global-function-names (be careful with this one - I think you'll need to import map)

For each of these rules, a PR is certainly welcome! To d

At the moment, it's possible to do

toc: true

to autogenerate a table of contents. I think it would be nice if there were also styling options, e.g. to have that TOC float on one side of the page.