Separate Block Chain Guide and Reference content

Live preview:
http://bitcoindevtest.us.to/en/developer-guide#block-chain
http://bitcoindevtest.us.to/en/developer-reference#block-chain-reference

This is an attempt at separating the "explain how it works" from the "describe specification" content on the Block Chain subsection.

- "Block contents", "Block headers" and some texts describing coinbase transactions are moved to the Reference page.
- Text about "Block height vs Block hash" is moved to "Block Height And Forking" and re-organized to become a natural progression in the existing text.