docs: publish a demo page to GitHub Pages

[lesnik512]

Adds a demo of the page at https://modern-python.github.io/release-scope/ for people who want to try it without GitLab or Jira.

How it is built

demo/scenario.py describes a made-up shop group with four services (api, web, worker, and admin, which is fully deployed) and six Jira issues. demo/build.py mocks GitLab and Jira with respx routes generated from that scenario and runs the real collect command, so the page and report.json always come from the current code and schema. A hand-written report.json would drift silently on every schema bump.

The scenario covers the states worth seeing: tags ahead of production, untagged changes, a failed and a running pipeline, staging deployments, an issue spanning two services, issues that ship anyway, and links that are open, closed, in production, or in a project outside the report. Dates are relative to build time, so the demo looks current. Every link points to example.com and goes nowhere.

Publishing

.github/workflows/demo.yml builds and deploys on each stable release tag, plus a manual trigger. Pages is already enabled for the repo (source: GitHub Actions), and the github-pages environment now accepts tags matching *.*.* as well as main.

Checks

• just demo builds into site/ (already gitignored); just demo-serve serves it on port 8000.
• A new smoke test builds the demo, checks the services and issues are listed, and chooses SHOP-38 to confirm the picks.
• demo/ is not in the wheel.