Add Telnyx messaging channel

[a692570]

Proposed changes:

• Added a built-in Telnyx Messaging channel for SMS and MMS assistants.
• Added Telnyx connector documentation and sidebar navigation.
• Added focused channel tests for inbound webhooks and outbound message payloads.

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog
• reformat files using black

Local checks run:

• uvx ruff check rasa/core/channels/telnyx.py tests/core/channels/test_telnyx.py
• python3 -m compileall -q rasa/core/channels/telnyx.py tests/core/channels/test_telnyx.py
• git diff --check

Note: I could not run the focused pytest file in a lightweight local environment because importing Rasa channel tests requires the repository full dependency set.

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