Move Contracts to a separate section

Temporary live preview here : (Merged)

It seems to me that contracts could be a standalone subsection focused on advanced transaction uses. The "Transaction" subsection is already loaded with content and it seems likely to me that various forms of contracts might be worth describing over time. Contracts can also imply additional offchain protocols (e.g. payment channels) which might be more or less relevant here.

This way, we could keep the Transaction subsection focused on describing what makes a transaction valid from the protocol standpoint, and separate transaction rules from advanced uses. 

In general, I've made few changes in the texts. This pull req:

- Moves ~~P2SH, Multisig and~~ Contracts to a separate subsection.
- Merges duplicate "Standard Transactions" points.
- Moves "Standard Transactions" right under ~~the introduction~~ P2PH.
- Mentions P2PH and "P2SH" before "Multisig", "Pubkey" and "Null" in "Standard Transactions"

(Note: this is an attempt and re-organizing things, perhaps it can be done in a better or different way or merged and further improved).