Add gateway peering examples to the VLAB demo section

[edipascale]

Fix #228

@Frostman I'm unclear whether we should also add documentation for external peering via gateway, seeing as we never documented virtual externals in the VLAB - if needed I can do both things in a separate PR

[github-actions]

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[Frostman]

@edipascale yeah, I think it's a good idea to add info on the virt externals and add examples of using them with both ExternalPeerings and gw Peerings. It could probably be a separate page under the VLAB section as existing ones are already quite loaded.

[Copilot]

Pull request overview

This PR adds comprehensive documentation for gateway peering examples in the VLAB demo section, including VPC peering with stateless and stateful NAT. Additionally, it removes the "experimental" designation from mesh topology references.

• Adds gateway peering documentation with practical NAT examples
• Updates CLI help text to include new options for gateway peering
• Removes "experimental" label from mesh topology feature

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
docs/vlab/demo.md	Adds new section on gateway peerings and NAT with practical examples, updates CLI help text for setup commands, includes examples of stateless and stateful NAT configurations
docs/vlab/running.md	Removes "experimental" designation from mesh topology description
docs/vlab/overview.md	Removes "experimental" designation from mesh topology description

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