docs: comprehensive README with release workflow and CI explanation

[gtrivedi88]

Summary

Rewrites docs/README.md to fully document the publishing workflow. Addresses questions like "does the build use docs from the release branch or from a tag?" and "how does the check job gate the build?"

What's new

• Cutting a new release section — step-by-step: create branch, update display_version, no CI changes needed
• How publishing works — detailed job-by-job explanation:
  • How check writes to $GITHUB_OUTPUT and build reads it via needs.check.outputs.is-latest
  • Explicitly states: docs are built from the release branch itself, no docs branch or tag involved
  • GitHub Pages configuration requirements (source, deployment branch rules)
• Flow diagram updated to show the build source
• Directory structure updated to include toolbar.hbs
• Branch table expanded with more context

What's removed

• Old "When a new release is cut" section (replaced by "Cutting a new release")
• Vague "no configuration needed" statement (replaced with specific steps)

Test plan

• README renders correctly on GitHub

🤖 Generated with Claude Code