Make doc headers clickable again

...without preventing clicks on other things.

Just use an `<a name="...">` tag that doesn't take up any space to make sure that we're not covering up something else.

For whatever reason, doing `position: relative; top: -$navHeight;` doesn't work and causes the anchor target not to be moved up. This solution works in both Chrome and Firefox.
Merge pull request #539 from spicyj/doc-anchors-again

Make doc headers clickable again