add warning about envoyfilter use in rate limit doc

[ilrudie]

Description

It has been noted that our rate limit doc may be perceived as suggesting EnvoyFilter as a stable API because it does not warn users about the inherit instability of EF. Add a warning to this effect for clarity in the "Before you begin" section.

Reviewers

• Ambient
• Docs
• Installation
• Networking
• Performance and Scalability
• Extensions and Telemetry
• Security
• Test and Release
• User Experience
• Developer Infrastructure
• Localization/Translation

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