Add references document defining NGF permissions

[bjee19]

Split out of #1976 , add a references document which defines NGF permissions. Can follow https://docs.nginx.com/nginx-service-mesh/reference/permissions/.

Should explain the permissions that the NGF Deployment requires (mostly security context options and required volumes).

Acceptance

• A document is added to NGF documentation to outline what permissions are needed by each component of NGF in order to run.
• The documentation explains permissions needed with the context of the data and control plane being split.
• Should include:
  • read-only filesystem
  • capabilities
  • non-root
  • volumes used

[mpstefan]

The result of this doc should help give a conclusion to this discussion: #1961

[bjee19]

User from community commented on #1674

Would be great to have this documented and why we need these extra capabilities, especially the KILL.