BIP-110: Clarify rule 2, update Deployment, and specify GBT usage

Updates to match the reference implementation and address community feedback:

• Rule 2: Several people were confused about which witness stack elements are limited by rule 2, so I have added a definition for "script argument witness items" (@murchandamus' preferred nomenclature) with a FAQ entry explaining exactly which elements are excluded and why. See this thread on the original PR for more context.
• Deployment: The reference implementation now has an EXPIRED terminal state in the state machine. Updated the deployment section to reflect this.
• GBT: Added a GetBlockTemplate subsection to the specification, following BIP 9 conventions, since the reference implementation now includes full GBT support.