Release Guide Pages for 3.0

[thunderbiscuit]

Keeping track of the features we want to highlight in the 3.0 Release Guide.

• Persisting locking of UTXOs feat!: Persist utxo lock status bdk_wallet#259
• Wallet::update returns Events Return wallet events when applying updates and blocks (3.0 milestone) bdk_wallet#319
• Removal of the TxBuilder in favor of Wallet::create_psbt Implement create_psbt for Wallet bdk_wallet#297