chore: publish the README to GitHub Pages (re-land #30 on main)

[hexember]

Why

PR #30 (the Pages workflow) was merged into its stacked base branch chore/readme-restructure, not into main. #29 had already been squash-merged into main about 20 minutes earlier, and GitHub didn't retarget #30. So main has no pages.yml, no deploy has run, and https://hexember.github.io/active-browser/ still shows the old README with the activebrowser.app install command.

This PR re-lands #30's exact diff onto main (git diff origin/main 14db2ce): .github/workflows/pages.yml, _config.yml, and the related CONTRIBUTING.md, Project.md and task-file edits. It adds nothing new.

After merge

1. Actions → pages → the run for the merge commit should be green (build and deploy).
2. After about 2 minutes, https://hexember.github.io/active-browser/ shows the current README with the View on GitHub button and the raw.githubusercontent install command.
3. curl -s https://hexember.github.io/active-browser/ | grep -c 'activebrowser\.app' prints 0, and curl -s -o /dev/null -w '%{http_code}\n' https://hexember.github.io/active-browser/install.sh prints 404.

The PR check on this branch runs the same Jekyll build and allowlist check as #30 did.

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