website: fix duplicated origin in generated sitemap URLs

[HarshPopat23]

What this PR does / why we need it:

Fixes #4455

The production sitemap at https://www.kubeflow.org/sitemap.xml contained malformed URLs with a duplicated origin prefix (e.g. https://www.kubeflow.orghttps://www.kubeflow.org/...).

This happened because layouts/sitemap.xml prepended https://www.kubeflow.org to {{ .Permalink }}, even though Hugo already generates .Permalink as an absolute URL based on --baseURL during Netlify builds.

This PR:

• Replaces <loc>https://www.kubeflow.org{{ .Permalink }}</loc> with <loc>{{ .Permalink }}</loc> in layouts/sitemap.xml.
• Adds {{- if .Permalink }} check to prevent emitting empty <url> blocks.

Which issue(s) this PR fixes:

Fixes #4455

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[google-oss-prow]

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[Arhell]

/ok-to-test

[HarshPopat23]

cc : @jaiakash @chasecadet @varodrig

[jaiakash]

Thanks for this @HarshPopat23
/lgtm

cc @christian-heusel @thesuperzapper