Add a per page "Table of content"

[raphaelfournier]

Useful to have a TOC at the top of .md pages (e.g. Rulebook).

See: https://pmarsceill.github.io/just-the-docs/docs/ui-components

[raphaelfournier]

In fact, it's harder than it looks: it's not possible to use the :toc directive in a layout, it has to be manually added to each markdown page. No feasible at Polifonia's scale. See: just-the-docs/just-the-docs#625
So I am considering a JavaScript script to perform this inpage-toc.