docs: add fallback and receive function documentation for Stylus Rust SDK 0.7.0+

[chainide-agent-bob]

This PR adds documentation for the fallback and receive functions introduced in Stylus Rust SDK 0.7.0:

• Added a new section in the Stylus Rust SDK guide explaining how to implement fallback and receive functions
• Updated the FAQ entries regarding fallback functions in both the troubleshooting partial and the JSON file
• Clarified that older SDK versions (pre-0.7.0) need to use workarounds, while 0.7.0+ has native support

Fixes: #2092

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