Write docs for UCP

[termontwouter]

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Copying from woutslabbinck/ucp-enforcement#8:

• How to install
• How to set up the ucpEnforcement Engine
• have ucon rules store
  • subsection: two options
  • have n3 interpretation rule
  • initialise plugins and policyexecuter
  • BOTH explain instantiation and without (write down the differences)
• Explain validate? (currently not part of package)