WIP: Add GPU plugin README prerequisites section

[eero-t]

This was inspired by ticket https://github.com/intel/intel-device-plugins-for-kubernetes/issues/1109, and fills one of the items listed in ticket https://github.com/intel/intel-device-plugins-for-kubernetes/issues/1060, by documenting how to get suitable kernel and user-space drivers. It also slightly updates other related sections.

Note: Kernel driver installation section is quite large because currently there does not seem to be any official documentation on how to install Intel discrete GPU kernel drivers. When there is, that section needs rewrite.

For now, I put installation of both kernel driver (= cluster admin responsibility), and user-space drivers (= WL developer responsibility) under prerequisites section, although it could be argued that latter could belong also to a (currently missing) "GPU plugin usage" section [1].

PS. I looked into splitting prerequisites and usage parts out of the fractionalization install section, but gave up. I think @uniemimu or @tkatila are the the only ones who can do that. After those are split out, there's actually some meat for the "GPU plugin usage" section.

[1] ticket https://github.com/intel/intel-device-plugins-for-kubernetes/issues/1045 asks for role-based documentation split-up.

[codecov-commenter]

Codecov Report

Merging #1149 (b9d1c43) into main (63481a2) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1149   +/-   ##
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  Coverage   52.99%   52.99%           
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  Files          40       40           
  Lines        4374     4374           
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  Hits         2318     2318           
  Misses       1927     1927           
  Partials      129      129           

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[eero-t]

Rebased to "main" to resolve conflicts with recently merged documentation changes.

[eero-t]

Better to merge this as is, and add information about DKMS when it actually becomes available. I made the GPU info in example outputs consistent, and squashed all changes to a single commit.