Fix GitHub Actions docs build by correcting broken docs references and sitemap hostname

[Copilot]

The build job failed during VitePress bundling due to unresolved documentation references, then surfaced additional config/link issues that still blocked a successful site build. This PR corrects those blockers in docs content and VitePress config.

• Asset path resolution

  • Updated docs/finalizing-setup.md to use the correct public asset path format for popups.png.

• Dead-link cleanup in setup guide

  • Removed two broken updating-electrode links in docs/guide/sd-preparation.md.
  • Replaced them with actionable support guidance (Discord) to avoid unresolved internal links.

• Sitemap URL normalization

  • Updated VitePress sitemap hostname in docs/.vitepress/config.mjs to a valid absolute URL.

- ![Electrode popups](assets/img/guide/popups.png)
+ ![Electrode popups](/assets/img/guide/popups.png)

- sitemap: { hostname: 'guide.samtendo.net' }
+ sitemap: { hostname: 'https://guide.samtendo.net' }

[sam51210]

No.