docs: expand and modernize the troubleshooting-transactions.mdx

[hongphuoc5635]

What changed? Why?

• This fits perfectly within the existing structure (no new top-level sections or major navigation changes), improves an active page, addresses common developer pain points (based on general L2 troubleshooting patterns and Base-specific features like minimum base fee/Flashblocks), and follows all guidelines. It enhances AI-friendliness, adds runnable examples, cross-links, and uses proper Mintlify components.

• Transaction issues are a top friction point for new Base developers (nonce problems, fee miscalculations, gas limits, confirmations). Updating with complete examples, verification steps, and ties to Flashblocks/min base fee makes it more actionable.

Notes to reviewers

• Update "Max Fee Too Low" section with a full code example using viem/ethers (popular for Base).

• Update "Transaction Included But Failed" section with example with simulation and error handling

• Expand Flashblocks/confirmations with integration example.

• Add "Querying Current Fees Programmatically" for proactive troubleshooting.

• Update Getting Help and add verification steps everywhere.

[cb-heimdall]

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