UEFI vs BIOS warning in upgrade documentation and troubleshooting guide

[yblatti]

Disclaimers : 
- I'm a hypervisor noob
- I'm not a native English speaker

While upgrading some XenServer 8.2 machines to XCP-ng using the ISO, one of them didn't offer to upgrade.
I didn't know that this machine was installed using Legacy BIOS instead UEFI.
I shamefully admit that it took me a veeeeery long time to understand what was wrong.

This PR proposes to add warning in the upgrade doc and a hint in the troubleshooting guide.

Also the "Upgrade via installation ISO" procedure linked to the download page for installation steps, which seemed to be wrong.

Before submitting the pull request, you must agree with the following statements by checking both boxes with a 'x'.

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

[stormi]

There's a trailing \.

But that's not the comment I wanted to make initially: mismatching firmware mode is not the only reason why the installer will not offer to upgrade in some cases.

For example, in some cases, the firstboot script in the previous installation never completed, which prevents the installer from detecting the existing installation.

@ydirson can likely tell more about it.

[yblatti]

Thank you for your feedback !

[...] mismatching firmware mode is not the only reason why the installer will not offer to upgrade in some cases.
For example, in some cases, the firstboot script in the previous installation never completed, which prevents the installer from detecting the existing installation.

TBH, this is out of my league. Feel free to add any relevant information in this section. We could also change the title to a more specific one.

There's a trailing \.

The new line was added for readability. Should I remove it ?