instancetypes: Add PreferredCPUTopology and SpreadOptions docs

[lyarwood]

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[lyarwood]

/cc @aburdenthehand

Andrew if you have time would you mind taking a look at this? Once I get back from PTO I'd also like to restructure the instance type docs and potentially move them into a sub directory with an independent OWNERS file if acceptable to folks.

[0xFelix]

@phoracek PTAL, thanks!

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