Add preview deployment to PR workflow

[Copilot]

Extend the PR build workflow to deploy a staging preview of the site using GitHub Pages preview deployments.

• Add pages: write and id-token: write permissions
• Upload build artifact via actions/upload-pages-artifact@v4
• Add deploy-preview job using actions/deploy-pages@v4 with preview: true
• Per-PR concurrency group (preview-${{ github.event.pull_request.number }}) with cancel-in-progress: true
• Preview uses a separate github-pages-preview environment to avoid interfering with production

Original prompt

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This section details on the original issue you should resolve

<issue_title>Build preview on PR</issue_title>
<issue_description>Extend the PR workflow to build the website so that we can preview it before publishing. Preview should produce a full website on a staging environment.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Build preview on PR #44

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[garrytrinder]

Is this attempting to deploy a pull request as a GitHub Pages preview site? (NOTE: This feature is only in alpha currently and is not available to the public!)

Source: https://github.com/actions/deploy-pages?tab=readme-ov-file#inputs-

Need to confirm whether the preview functionality was released to the public. If not, then an alternative would be to deploy to another repo configured with GH Pages.