Where is the official developer documentation provided by EOS?
The question asks for official Documentation sources which has already been answered by lucca65
Although not an official source, it might be worth checking out https://www.eosdocs.io/
They are "an Open Source project which encourages the community to contribute to exceptional EOS documentation." I feel this is important to mention as the spirit of EOS is very much about community and mutual contribution. In my opinion the documentation there has already exceeded the official sources.
It appears that the official documentation listed above is now deprecated. According to EOSIO GitHub
The wiki is deprecated and will be removed June 30th. Please check out the constantly improving developers portal
The developers portal can be found here: https://developers.eos.io/