Make navigation sidebar sticky using CSS

[rikvl]

Fixes issue #11, but the behaviour for a long ToC (that overflows the element) is a bit different.

[mhvk]

@astrofrog - would you, or someone else, be able to review this PR? We're using it in one of my packages now and it is quite nice! @rikvl and I just had a look at the two competing options, and agreed this is the nicest one (so we closed #23).

[stevewhitmore]

👍 for this PR. @rikvl mentioned this here and while it may not be a perfect solution it's a step in the right direction. Would it be possible (or a reasonable request) to make the nested bulleted items collapsible? That would solve the overflow problem. I'm asking about this blind and I'm not very familiar with this code base. I came searching after being annoyed all day with having to scroll all the way back to the top in the Python docs.