Add link to Developer Documentation in dev-ref and dev-guide

Live preview: http://bitcoindevtest.us.to/en/developer-guide

In response to @harding comment https://github.com/saivann/bitcoin.org/pull/117#issuecomment-42472812, (about dev-ref and dev-guide pages not linking to the portal page), I think this is probably the most useful and simple solution. Displaying the link at the end of the toc makes it available at any time when scrolling the page while not catching too much attention.