[Task] Add some documentation for log level configuration

[kunalk16]

📝 Objective

Help users identify how to change the log level through the configuration.
Specifically https://github.com/sipeed/picoclaw/blob/main/config/config.example.json#L471

📋 To-Do List

• Add some information in the config documents on how to control log level

🎯 Definition of Done (Acceptance Criteria)

• Documentation is updated in the README/docs folder.
• Code follows project linting standards.
• (If applicable) Basic tests pass.

💡 Context / Motivation

This is not a bug: #2006
Lack of information may have confused the user

🔗 Related Issues / PRs

• Fixes #
• Relates to feat(logging): add configurability for log levels preference #1853 chore(config): move loglevel settings under gateway #1912

[kunalk16]

@Alix-007 since you have worked on the PR, could you please review the comment that I have posted on your PR?