Document the Block Kit maintenance workflow

[nicklambourne]

Summary

• Add a maintenance guide for new Block Kit types, fields, vocabulary values, validation rules and composition helpers.
• Cover official Slack evidence and validator probes, shared contract changes, native APIs in every maintained language, generation order, conformance/export coverage, SDK integration, documentation and coordinated release readiness.
• Include a worked example using existing image slack_file support and a copyable PR evidence checklist.
• Link the guide from the root README, contributor page, shared specification and repository agent instructions; correct the contributor page's release skip-list guidance.

Dependency

#378 has merged. This PR now targets master and contains only the five documentation/guidance files. The branch was updated with master without changing the previously validated file contents; refreshed CI is running.

Verification

• Verified all 36 explicitly named repository paths and the worked-example model/fixture IDs.
• Checked generation commands and language-specific gates against the existing generators and workflows.
• Generated all six language API references; guide, language-content and Ruby snippet checks passed.
• Docusaurus production build passed using an explicit scratch output directory.
• All post-build checks passed: language-scoped search, version selector, six-language API rendering, legacy fixtures/contracts and release snapshots.
• Checked the rendered guide's headings, contributor navigation, check links and current-only version placement.
• git diff --check passed; working tree was clean after removing temporary build links.

This changes documentation and maintainer guidance only. Package behavior, spec versions, workflows and historical documentation snapshots are unchanged.