Twenty Seventeen: Table of contents numbers

[sami.keijonen]

Should table of contents have numbering before the release?

[karmatosed]

I've added a patch with the changes to table of contents. @davidakennedy if you can take a look please. Thanks @sami.keijonen for mentioning we should do this.

[helen]

This seems fairly low priority to me, so take that into account as I throw this question out there - what is the benefit of numbering things? Seems like a maintenance burden and searching for numbers alone isn't really all that great of an index in a CSS file, so you end up typing some amount of the title string anyway.

[davidakennedy]

In 39216:

Twenty Seventeen: Adds numbers to the table of contents in style.css

Props karmatosed.

Fixes #38767.