Update the Guest UEFI Secure Boot guide

Update for XCP-ng 8.3 while retaining compatibility with XCP-ng 8.2.1.
Reorganize.
Add a Troubleshooting section that Xen Orchestra will link to.
Add a link towards the guide in the VMs page.

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[x] "I accept that my contribution is placed under the CC BY-SA 2.0 license [1]."

[x] "My contribution complies with the Developer Certificate of Origin [2]."

[1] https://creativecommons.org/licenses/by-sa/2.0/ [2] https://docs.xcp-ng.org/project/contributing/#developer-certificate-of-origin-dco

xcp-ng / xcp-ng-org

Update the Guest UEFI Secure Boot guide #273