Make each header linkable with an on hover icon

[Foxandxss]

/cc @ericjim

From the style guide team we thought that it would be good to have each header linkable so we could easily share URLs for concrete styles, something like what Github does:

Instead of making each header a link, better a hidden SVG icon on the left that would appear on hover.

Then we thought that it would be good to have that in the entire documentation.

[naomiblack]

paging @alexwolfe -- would love some design input from you if you have any thoughts

[ericjim]

We sort of do something like this already on the api docs, https://angular.io/docs/ts/latest/api/common/index/NgSwitch-directive.html#!#ngSwitch-anchor, you'll notice there is a section anchor on the link which leads directly to the specific section in Class Details

[KyleBastien]

Headers can currently be linked to, which is good, but in my opinion it's very difficult to discover. Adding an on hover icon, or something to allow the user to better understand that the Header can be linked to, and what that link is, allows for ease of sharability. Hopefully, that encourages people to share and discuss the docs more. This was at least my original idea when I opened #1397.