Document Function and JobFunction memory limits

[dragon3]

Summary

• Adds Function (32 MB) and JobFunction (256 MB) memory limit rows to the Service Limits table in docs/reference/platform/platform-limits.md
• Adds a new "Memory Limits" section describing the fixed, non-configurable limits and the Memory limit exceeded error (e.g., used 50 MB of 32 MB limit)
• Adds two best-practices bullets: design for memory limits, and prefer JobFunction for memory-intensive workloads