Merging old x86/x86_64 documentation from Sourceforge into docs

[samveen]

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As per docs/source/guides/admin-guides/manage_clusters/x86_64, any documentation for x86_64 clusters points to the sourceforge documentation at https://sourceforge.net/p/xcat/wiki/XCAT_Documentation/

I'd like to migrate and merge it into the main documentation, so that it can be kept up to date to xCAT current version. If that's OK, I'll create a draft pull request for the merge.