Reconcile README with User Guide and fix links broken on the docs site

[Cito]

The README doubles as the landing page of the great-docs site, which caused two problems: repo-relative links (./examples, ./src, ...) resolved on GitHub but 404ed on the site, and the content duplicated large parts of the User Guide.

• Convert all repo-relative links to absolute GitHub URLs so they work in both contexts (a hidden comment in the README now documents this constraint)
• Replace the outdated developer-cookbook link with the User Guide's Architectural Concepts chapter
• Trim the architecture narrative and getting-started sections, deferring depth to the User Guide
• Move the development-environment and docs-build sections to a new CONTRIBUTING.md, linked from the README
• Refresh stale content: the protocol/provider table now includes key-value stores and the outbox publisher, and the extras list matches pyproject.toml

[coveralls]

Coverage Report for CI Build 29341081100

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Coverage: 93.325%

Details

• Patch coverage: No coverable lines changed in this PR.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

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Coverage Stats

Relevant Lines:	3970
Covered Lines:	3705
Line Coverage:	93.32%
Coverage Strength:	0.93 hits per line

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💛 - Coveralls