chore(pages): replace JamesIves/github-pages-deploy-action with actions/deploy-pages

[chuanye-gao]

Closes #133

What does this PR do?

This PR replaces the third-party
JamesIves/github-pages-deploy-action with the official GitHub Pages
deployment workflow based on:

• actions/upload-pages-artifact
• actions/deploy-pages

The demo ADR site is still generated using log4brains build, which aggregates
all ADR sources declared in .log4brains.yml into a single static site.

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Why this change?

GitHub now recommends deploying Pages sites via Actions artifacts instead of
pushing generated files to a dedicated branch.

Migrating to the official Pages Actions:

• Removes the dependency on a third-party deploy action
• Keeps generated static files out of Git history
• Clearly separates build and deploy steps
• Reduces the permissions required by the CI workflow

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Implementation details

• Added a GitHub Pages workflow that:
  • Builds the demo site with log4brains build
  • Uploads the generated output (.log4brains/out) as a Pages artifact
  • Deploys the site using actions/deploy-pages
• The workflow is triggered only when ADR inputs (as declared in
  .log4brains.yml) or generation logic change.

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Notes on migration

This PR does not remove or modify the existing gh-pages branch deployment.
After merging, the Pages source can be switched to “GitHub Actions” in the
repository settings.