Change named headers into subtle links to themselves

Many sections in the language reference have page anchors, but to know the anchor name one has to look in the source code, effectively rendering them useless.

This change make these headers links to themselves, so people can easily share links to *specific* parts of a page.

The links have subtle styling. When not hovering over one, it looks like any other header. Hover over it, and it gets an underline and a pilcrow/paragraph symbol after it.