Expand P2Pool share-chain guidance

[ryzhkevichpavel-del]

Summary

Expands the mining guide's P2Pool coverage using the direction suggested in bitcoin-dot-org/Bitcoin.org#1168.

This PR adds:

• a fuller explanation of how P2Pool reduces variance without central pool custody
• the share-chain concept and how it parallels Bitcoin's block-chain selection by cumulative proof of work
• direct-payout behavior through coinbase commitments in shares
• two diagrams adapted from the older mining guide material referenced in the issue discussion
• brief tradeoffs and resource links

The goal is to move beyond a short text-only mention and provide something closer to the original P2Pool section maintainers pointed to in the archived mining guide.

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

Bounty

This PR is submitted for the bounty attached to bitcoin-dot-org/Bitcoin.org#1168. If accepted, BTC payout can be sent to:

bc1qewlnv3xw3rtll8c5rxjse0fl6v36kkxvwfvccr

Validation

• git diff --check passed
• Confirmed the new SVG assets do not contain <script> tags
• Full Sphinx build was not run in this environment

[ryzhkevichpavel-del]

Local verification update: I built the docs site successfully in a clean copy of the repository using Sphinx on Windows after excluding only local helper/temp directories created during verification. The HTML build completed successfully and the only remaining warnings were pre-existing, outside the files touched by this PR (the repo's language = None config warning and an existing glossary formatting warning in glossary.rst). The updated devguide/mining.rst and the two added diagrams rendered and copied into the build output successfully.