[codex] Refresh the beginner Solana basic feed guide

[tamlut-modnys]

Summary

This rewrites the Solana basic feed guide around a true beginner quickstart and aligns it with the verified example flow.

What changed

• keep the happy path focused on fetching a single BTC/USD feed
• add a clearly labeled SOL/USD override path
• standardize feed discovery guidance around Explorer search and bare 64-character hex feed IDs
• explain that feed.value() is already the human-readable decimal value
• align the optional consumer-program instructions with the local build and deploy flow used by the example repo
• remove generalized multi-feed framing from the main tutorial path and move that complexity out of the way

Why

The previous guide assumed more context than a new user had and no longer matched the example code closely enough. This version reflects VAYEN's feedback and the now-verified example workflow.

Validation

• matched the guide against the updated sb-on-demand-examples basic feed implementation
• verified the example's default BTC/USD flow end-to-end on devnet
• verified the SOL/USD override flow end-to-end on devnet
• verified the optional consumer-program path after deployment on devnet