[Draft-Extensible-Tx]: Add protocol bundles introduction and bundle type registration process.

Restructure the specification to begin with a comprehensive introduction to
protocol bundles, explaining effecting data, authorizing data, and the
transparent transaction value pool concepts. Define the bundle type
registration process that ZIPs must follow when introducing new bundle types.
Move the bundle type registry to appear before the transaction format
specification for better readability.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: nuttycom