Add anchor id when hovering a header

[Ainali]

The theme is nice and creates and id for each heading which makes it possible to link to any header by using it as an anchor. However, the only way to see it is to view source.

It would be nice to do something similar to how GitHub or HackMD does, and show a 🔗 in the left margin when hovering over the header which has that link so that it can easily be reused elsewhere.