Add headline hover-anchors for copying links to headlines

[Kissaki]

Probably needs custom markdown rendering that includes the link, and CSS to hide it by default but show it on hover.

[Kissaki]

Implemented in ad5f7e1 for the most part.

Does not include all types of headers. The regex should be updated to match all head tags. But then the 1.3.0 release blog post layout has to be adjusted for the <details><summary><h headlines.