Add bridge mode warning: untagged external networks incompatible with…

[sethesterline]

… Physical Bridge Mode

When Physical Bridge Mode is enabled on a physical network, all traffic must be VLAN-tagged. External networks with VLAN type None (untagged/native) will not pass traffic on a bridged interface. Adds a warning callout to the trunk mode Virtual Switch Port setup section and a bridge mode info note to the Networking Concepts physical network definition.

Closes #461

[jasnoyaeger]

@sethesterline — flagging one open question before merging (also sent over Slack):

The callout added to network-concepts.md lists both Virtual Switch Ports and Tenant Layer 2 Networks as use cases for the Physical Bridged toggle, but product-guide/tenants/layer-2-networks.md never mentions Physical Bridge Mode as a prerequisite.

Two possibilities:

1. Tenant Layer 2 Networks does require bridge mode under the hood → layer-2-networks.md is missing that prerequisite and should get the same untagged-traffic warning.
2. Tenant Layer 2 Networks doesn't use bridge mode → the network-concepts callout is slightly misleading, and we should reword the link as a "related feature" rather than a "use case for bridge mode."

Which is it? Happy to follow up with edits either way.

[bcampbellverg]

Currently working on reviewing this doc - discussing with development and setting up a test environment to walk through scenarios. I think it may be possible that bridge mode does not actually require all traffic on the interface to be VLAN-tagged --- but a reboot might be necessary after the network is changed to bridged mode. I will work to verify if this is the case or not.