Auto-scroll in devel-docs table of content

Live preview: http://bitcointest2.us.to/en/developer-reference#getinfo

As suggested in by @harding on the mailing list:
https://groups.google.com/d/msg/bitcoin-documentation/s_mL1syxYoQ/-WJAj5RmHvIJ

This allows the table of content to auto-scroll in order to keep the active toc entry visible when the user scrolls the page.