Update Durable Nonces Course (#450)

* Update Durable Nonces Course

- Fix grammatical mistakes
- Partial signing for transactions with multiple signers.
- Use `getExplorerLink` from solana-helpers for easier link generation.
- Added the calculation for the exact rent-exempt balance using `connection.getMinimumBalanceForRentExemption`.
- Added screenshots of successful test runs are included in the documentation.

* Addressed review feedback

- Improved explanation of signatures to avoid circular logic
- Updated the "Considerations" section to maintain a consistent perspective
- Added detailed points to educate users on mitigating risks associated with durable nonce transactions
- Cleanup unnecessary word
- Fixed terminology: changed "off-chain" to "offchain" throughout the document
- Clarified terminology: changed `nonce account` to nonce account

Author: chandrapratamar