Disable BGP propagation guidance

[victorar]

Current ESLZ guidance for hub and spoke network architecture when connectivity between landing zones across two hubs is required, is to cross-connect the ExpressRoute circuits across hubs.

While this scenario addresses the required connectivity needs, spokes across hubs will communicate directly bypassing the firewall. We need to update the guidance to indicate that, if customer requires traffic across hubs to be inspected by their firewall, they have to either 1) create more specific route entries in their UDRs (so that traffic across hubs is redirected via the local firewall) or 2) disable BGP propagation on the spokes, as this will simplify the route configuration in the UDR.

[sitarant]

Amended guidance on hub and spoke
https://github.com/MicrosoftDocs/cloud-adoption-framework-pr/pull/2007

[victorar]

Closing issue as PR has been merged.