docs: replace placeholder production URL in Docusaurus config

The current Docusaurus configuration still uses the starter deployment URL:

- `skel/docs/docusaurus.config.js:11` sets `url` to `https://your-docusaurus-test-site.com`.
- The same config is used for the Deepfence ThreatMapper documentation site, and the repository has an open site issue plus an existing history of deployment/domain work.

Because Docusaurus uses `url` when generating canonical URLs, sitemap/SEO metadata, and absolute links, the published site can emit links pointing at the placeholder host. Could this be replaced with the actual documentation host (for example, the currently supported `docs.deepfence.io` host if that remains canonical), and then verified by inspecting a generated page’s canonical URL and sitemap?

Start with skel/docs/docusaurus.config.js:11 and confirm which documentation host is currently canonical, checking the open site issue and existing deployment/domain history. Update the configured URL, then generate the site and inspect a page’s canonical URL and the sitemap to verify they use the selected host.

Written by the indexing model from the issue text.