docs: add troubleshooting guide for non-standard ERC-20 token approvals

[iborazzi]

Summary

This PR adds a dedicated Troubleshooting section to the main documentation (README.md) addressing common issues with non-standard ERC-20 token approvals on Ink mainnet (such as approve(spender, 0) failing with an "internal error" due to specific token contract implementations).

Problem Addressed

• Users interacting with certain non-standard tokens (like specific mainnet tokens where direct resetting to 0 fails while an active allowance exists) often encounter unexpected transaction reverts or wallet "internal error" warnings.
• Providing a clear troubleshooting note helps developers and users understand whether the behavior stems from contract-level specifics rather than network/infra issues.

Changes Made

• Added a new subsection: Troubleshooting: Non-Standard ERC-20 Tokens & Approvals.
• Outlined potential contract implementation nuances.
• Provided actionable workarounds (such as setting an intermediate lower allowance value or inspecting token interfaces via Blockscout).