Add multi-branch GitHub Pages publishing for rendered ZIP docs

[Copilot]

This PR switches Pages publishing from a single-branch deployment model to branch-aware rendering so each branch can publish its own all-zips output under a branch path. It integrates the repository with shielded-nate/multi-branch-pages and removes automatic conflict with the legacy publisher.

• Multi-branch Pages workflow

  • Adds .github/workflows/multi-branch-pages.yml.
  • Calls shielded-nate/multi-branch-pages reusable workflow (pinned by commit SHA).
  • Configures branch discovery (branch_whitelist: "*") and publishes rendered/ output per branch.
  • Uses nix develop --command make all-zips as the branch render command.

• Legacy deployment conflict removal

  • Updates .github/workflows/publish.yaml trigger from push: [main] to workflow_dispatch.
  • Keeps legacy workflow available for manual use while preventing concurrent automatic Pages deployments.

jobs:
  publish:
    uses: shielded-nate/multi-branch-pages/.github/workflows/publish-multi-branch-pages.yml@7877d18060b344723fdafd145aeeaee6ee9e7361
    with:
      branch_whitelist: "*"
      render_command: nix develop --command make all-zips
      render_output_dir: rendered