docs: add generated Sourcey API documentation

[q10283245]

What changed

• add a pinned Sourcey 3.6.5 configuration that generates reference pages from both the v1 and v2 Go modules
• add concise guides for version selection and common workflows
• add a GitHub Actions build on documentation and Go changes, with Pages deployment after changes land on master

Why

The repository currently points users to source and package docs in several places, but it does not have one searchable reference covering both module generations. The generated site keeps symbol documentation tied to the repository source while separating v1 and v2 clearly.

Validation

• npm ci in docs/sourcey
• npm run build in docs/sourcey
• Sourcey generated 18 pages plus llms.txt, llms-full.txt, a search index, and sitemap
• spot-checked generated v2 symbols including NewBlockReader, OpenReader, NewSelectiveWriter, StoreIdentityCIDs, and ReadFrom

This is a draft so maintainers can first confirm whether the proposed documentation location and Pages deployment model fit the project.

[rvagg]

no spam pls