As an extension to https://issues.redhat.com/browse/OSSM-5634, we need to include documentation for creating an Egress gateway with OSSM 3.

This will be very similar to an Ingress Gateway, but with different Gateway configuration and should like live in the same doc page.

This is important because:

• OSSM 2 created an egress gateway for users by default and this is no longer true in OSSM 3
• Some community Istio profiles also create an egress gateway for the user

Put together, new OSSM 3 users may not realize that they need to create a gateway for egress, and we should include a sample configuration in Github and product documentation to help with this.

Upstream sail-operator issue: https://github.com/istio-ecosystem/sail-operator/issues/379