Docs: restructure the site and add contributor pages

[SandObserver]

Summary

• Reorganise the sidebar into Get started, Build your dashboard, Widgets, Run Stackyard, Settings, Troubleshooting, Create a widget, Contributing and About, with search-oriented page titles.
• Rebuild every widget page on shared ServiceList, SettingList and LinkList components: a live preview, the supported services with their fields and project links, and the settings.
• Add Create a widget (tutorial, manifest, data, widget page, checklist) with a live preview of the repository widget template, synced by sync-widgets.mjs.
• Add Contributing pages (how to contribute, architecture, design system, translations, API errors, releasing) and generated Security policy and Governance pages.
• Move environment variables onto the Docker page, add a reverse proxy page, fold the socket proxy setup into Badges, and add the SSRF range table to Security.
• Fix the cosign identity pattern to the repository's case.
• Redirect /docs/development/, /docs/adding-services/ and /docs/advanced-configuration/ with Astro redirects and public/_redirects.
• Add a light-background version of the mark.

Testing
npm run build and npm run check pass. Verified in Chrome in dark, light and forced light themes at 1280px and 400px, with no horizontal overflow.

Checklist

• npm run build and npm run check both pass.
• Product UI is shown by running or capturing it, never recreated in CSS.
• Any new preview or screenshot carries no private hostnames or real data.
• Visual changes verified in both colour schemes and both docs themes.
• Links to the app repo still resolve.

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