Add a temporary BETA disclaimer in devel-docs

Live preview: http://bitcoindevtest.us.to/en/developer-guide

This pull req adds a temporary disclaimer in devel-guide and devel-ref, mentioning that the documentation is in BETA. Along with the "Report an issue" button in the toc, I think these are reasonable measures to let us publish this documentation while it is partially reviewed by core developers.

The disclaimer is fixed-positioned so it appears even when people click on anchors, and it can be closed until the browser session ends (thanks @harding for the idea).