Fix dead links and update links that are now redirecting on Stanford XRootD webpages

[Macman1234]

This PR updates the https://xrootd.slac.stanford.edu/* webpage links in:

• The comments of metrics/xrootd_metrics.go and xrootd/xrootd_config.go
• The help fields of the Grafana dashboard docs/public/origin-dashboard-template.json
• The documentation pages CONTRIBUTE.md, docs/app/about-pelican/core-concepts/page.mdx, and docs/parameters.yaml

so that

• The dead links to https://xrootd.slac.stanford.edu/doc/[path-to-docs] point now to https://xrootd.web.cern.ch/doc/[different path], confirming that both the page and section links are updated and correct
• The links to the base page https://xrootd.slac.stanford.edu, which redirects to https://xrootd.org, directly point now to https://xrootd.org