Create Bitcoin Developer Documentation portal page

(As discussed on the mailing list)
Live preview here: http://bitcoindevsplit.us.to/en/developer-documentation

Feedback / requests / suggestions welcome!

Each category is mostly empty right now, so I guess we'll later add links to the "Developer Reference" page and maybe also some useful external links? e.g. http://www.righto.com/2014/02/bitcoins-hard-way-using-raw-bitcoin.html . If we agree with that, "Finding few good external links" could be added to the TODO list. The future "Developer Cookbook" page will also help, but as @harding suggested, let's ignore this for the first version.

I will push missing icons at a later time.