Add VM affinity docs in Capacity Planning

[mattrowe-opennebula]

Add the placement policy documentation to Product > Cloud System Administration > Capacity Planing > Affinity Groups.

Internal documentation: https://docs.google.com/document/d/1f_bnF2d5f0Tc-vPqjHF5ucI53j57Sou6x-U4Hdi_tl0/edit?usp=sharing