Doc: Replace absolute links with relative ones

[d3athjest3r]

Some links still contain old github paths and were not pointing to docs.xcp-ng.org. Replace those links with relative links to work even if the base URL changes.

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• "I accept that my contribution is placed under the CC BY-SA 2.0 license [1]."
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[1] https://creativecommons.org/licenses/by-sa/2.0/
[2] https://docs.xcp-ng.org/project/contributing/#developer-certificate-of-origin-dco