Docs: add anchors to sections, apply header offset for sections

[cesarandreu]

Two usability improvements suggestions for the docs:

1. Add anchor link to sections. Github does this.
2. Apply header offset on anchor links. For example, if you link to How to deal with my log-in page?, it will cover the section. This is easily solved by using the :before css selector and setting a height with a negative margin.

[juliemr]

This is done with the commits above.

[cesarandreu]

Awesome! Thanks, I think this really helps usability.

[avdd]

The character used for this (0x1f517) is not visible on Ubuntu 14.04.