Add interactive examples to the PDA, rent and fee chapters

[gymnasy55]

Proposal: three chapters of the Solana tutorial get a small interactive example right after the concept they explain, so a reader can poke at it instead of only reading. Each is a flow from PLGRND, an open-source visual playground for Solana (MIT, source: https://github.com/RedDuckTeam/PLGRND), embedded as one <iframe> line plus a one-sentence caption.

• PDA vs Keypair Account — before "Should I use PDAs or Keypair accounts?". Left: a keypair account signs a message and the signature verifies against its public key. Right: a PDA derived from the chapter's program id with empty seeds, and there is no private key to sign with. Same contrast the chapter makes, but editable.
• Cost of storage, maximum storage size, and account resizing — before "Comparing Storage costs to ETH". One branch asks a mainnet RPC node for the rent-exempt minimum of an account of the given size, the other computes (128 + bytes) × 3,480 × 2 with the chapter's constants, and an EQUAL node confirms they match. Change the byte count and both update.
• Compute Units and Transaction Fees — right after the bold line that the transaction charged 5,000 lamports regardless of compute units. Change the number of signatures and the fee moves in steps of 5,000 lamports.

Screenshots: