OnHover magically appearing links

[DanRathbun]

Thomas made note in another issue thread about liking these style links:
http://breathe.readthedocs.io/en/latest/#_CPPv2N8NutshellD0Ev
... so I started a new one to talk about link styling.

Basically they are onHover (onMouseOver) magically appearing links.
They'd work well for the permalinks.

[DanRathbun]

I have gotten the popup "permalinks" to work well. Normal rendering:

And then onHover anywhere over the h2.summary_header parent element:

I used the same styling as the "note title constructor" class indicators in the "Instance Method Summary" (expanded) lists. Ie:

... except that I created a new class named "permalink",
in an addendum stylesheet named "styles_add.css".
(It's better to leave the default stylesheet file as is from YARD master.)