docs: add RabbitMQ MQTT connection instructions to MQTT source page

[Copilot]

• Review the current MQTT source documentation structure
• Add a note section explaining RabbitMQ MQTT plugin connection
• Include the three setup steps: enable plugin, set feature flag, and connection readiness
• Add reference link to RabbitMQ MQTT documentation
• Verify the change matches the docs style guide
• Test spell check and formatting

Summary

Added documentation for connecting RisingWave to RabbitMQ using the MQTT protocol. The note includes:

1. Command to enable the RabbitMQ MQTT plugin (rabbitmq-plugins enable rabbitmq_mqtt)
2. Command to set the MQTT v5 feature flag (rabbitmqctl enable_feature_flag mqtt_v5)
3. Confirmation that users can then connect using the MQTT connector
4. Reference link to RabbitMQ's official MQTT documentation (https://www.rabbitmq.com/docs/mqtt)

The change is minimal and follows the existing documentation style, using the <Note> MDX component consistent with other sections of the docs.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Document: Instruction for connecting to RabbitMQ with mqtt protocol</issue_title>
<issue_description>You need to add a notice part on the MQTT SOURCE page, on how to connect to RabbitMQ instance, consume RabbitMQ topic with the help of its MQTT plugin.

operation on the broker side:

1. enable the mqtt pulgin rabbitmq-plugins enable rabbitmq_mqtt
2. set mqtt_v5 feature flag rabbitmqctl enable_feature_flag mqtt_v5
3. good to go </issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #745

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[Copilot]

Pull Request Overview

This PR adds documentation for connecting RisingWave to RabbitMQ instances using the MQTT protocol. The documentation provides essential broker-side configuration steps that users must complete before they can successfully use RabbitMQ as an MQTT broker with RisingWave.

Key changes:

• Added a new Note section explaining RabbitMQ MQTT plugin configuration
• Included step-by-step instructions for enabling MQTT support in RabbitMQ
• Positioned the documentation strategically after the Prerequisites section

[tabVersion]

@copilot you need to mention the doc link on the rabbitmq side https://www.rabbitmq.com/docs/mqtt

[Copilot]

@copilot you need to mention the doc link on the rabbitmq side https://www.rabbitmq.com/docs/mqtt

Added the link to RabbitMQ MQTT documentation in commit e8f4c3a.