Add P2Pool share chain guidance

[KingParmenides]

Summary

• Adds a P2Pool and share chain section to the mining developer guide.
• Explains how P2Pool tracks shares with a peer-to-peer share chain instead of central pool accounting.
• Describes direct coinbase payout commitments, variance reduction, and the decentralization/tradeoff profile.

Bounty

Addresses bitcoin-dot-org/Bitcoin.org#1168.

BTC payout address: 39Q34P8A7g375yqEr8buvNJkUbRgKfbKQZ

Testing

• git diff --check
• PATH=/private/tmp/codex-developer-bitcoin-venv/bin:$PATH make clean html

Build succeeded. Existing warnings remain for conf.py language=None and glossary.rst:647; no new warnings from this change.