Add NGINX Agent v3.0 SELinux configuration guide

[dhurley]

Proposed changes

Add NGINX Agent v3.0 SELinux configuration guide

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[mjang]

I see this is similar to https://docs.nginx.com/nginx-instance-manager/system-configuration/configure-selinux/#selinux-agent from our NIM docs.

I think we can also use the "include", similar to this line,
to make sure we stay consistent.

Do you also need content from https://docs.nginx.com/nginx-instance-manager/system-configuration/configure-selinux/#add-ports-to-nginx-agent-selinux-context

[JTorreG]

@dhurley what is the status of this PR? there is some feedback from Mike