[Task] Add reference to the SECURITY_CONFIG.md file in configuration.md

[kunalk16]

📝 Objective

Recently the configuration management refactoring was done and the secrets are to be moved from the config.json to security.yml folder.

The documentation for this exists(https://github.com/sipeed/picoclaw/blob/main/pkg/config/SECURITY_CONFIG.md) but there are no references in the user docs leading to that file, I had to do an LLM analysis of the repo to locate this file. Can we add a reference to this file in this file: https://github.com/sipeed/picoclaw/blob/main/docs/configuration.md

Optional to add the link to this document in the error message for missing api key, etc. from code.

📋 To-Do List

• Update the file https://github.com/sipeed/picoclaw/blob/main/docs/configuration.md to refer to the file https://github.com/sipeed/picoclaw/blob/main/pkg/config/SECURITY_CONFIG.md
• Optional: Add reference to this file in the code as well for missing api key, etc. errors.

🎯 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

I was trying to look at a github issue and I was unable to configure a fresh installation of picoclaw from the main branch. I had to do an LLM analysis of the repo to locate where to store the api key for version 1 config.

🔗 Related Issues / PRs

• Fixes #
• Relates to #