Strengthen backwards compatibility guidance in developer docs

What does this MR do and why?

This merge request updates the documentation on real-time features in GitLab, focusing on WebSocket usage.

It adds a new section on working safely with WebSockets, emphasizing the importance of backwards compatibility and graceful degradation. The changes stress the need for defensive coding and caution when introducing new WebSocket connections, especially at scale.

The update also reorganizes existing content, moving deployment-related information earlier in the document and providing more detailed guidance on estimating the impact of new features on WebSocket connections. Overall, these changes aim to improve the safety and reliability of real-time features in GitLab while providing developers with clearer guidelines for implementation and deployment.

References

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Quoting from this Slack thread (as they are ephemeral):

I’ve tested the notes with disabled subscription and the note isn’t added since we don’t have a logic to update notes after the mutation, we fully rely on GraphQL subscription for that.

In short, a feature was built relying solely on WebSockets, which malfunctioned when they weren't available.

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