[ADD] phone: call flow designer

[huisit]

What this PR does and why it's needed

Documents the new Call flows feature in the Phone app: a visual, node-based builder for routing inbound calls to users, groups, queues, menus, voicemail, or external numbers. Adds a new dedicated page and wires it into the Phone app's landing page alongside the existing Phone widget and Devices/integrations pages.

Changes

Phone app documentation

• Added phone/call_flows.rst, covering:
  • Configuration prerequisites (purchasing a phone number, and the supporting PBX resources — Extensions, Groups, Queues, Menus, Voice Mailboxes, Sounds/Music on Hold, Time Conditions)
  • The full list of node types available in the call flow builder (Call a User/Contact/Group, Send to a Queue, Open a Menu, Play Audio, Send to Voicemail, Redirect to an Extension/External Number, Time Condition, Hang Up)
  • The step-by-step process for designing a call flow and assigning it to a phone number
  • A Troubleshooting section covering three edge cases: no phone numbers available, a Time Condition with no open period configured, and stale queue/group node data
• Added a "Call flows" card to the Phone app landing page (phone.rst), next to the existing "Phone widget" and "Devices and integrations" cards, and added the corresponding toctree entry.

Note

Most of the Configuration navbar items are completely new. This article will be updated with ref links to those items as the respective articles are added.

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This master PR can be FWP up to master.

[robodoo]

[huisit]

@stepperpig this is ready for review when you have a moment!