Add Custom data streams docs

[alexandra5000]

What does this PR do?

Introduces a new page describing advanced data stream routing in EDOT. The page explains when routing customization is appropriate, the recommended pattern of using data_stream.namespace, the risks associated with modifying data_stream.dataset, and the naming constraints for template-based routing.

Why is it important?

Until now, this information existed only in a KB and was not discoverable in the official documentation.

Checklist

• I have read and understood the pull request guidelines of this project.
  - [ ] My code follows the style guidelines of this project
  - [ ] I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files
  - [ ] I have added tests that prove my fix is effective or that my feature works
  - [ ] I have added an entry in ./changelog/fragments using the changelog tool
  - [ ] I have added an integration test or an E2E test

Disruptive User Impact

None

Related issues

Closes #4133

Generative AI disclosure

1. Did you use a generative AI (gen AI) tool to assist in creating this contribution?

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Claude 4.5 Sonnet via Cursor

[github-actions]

🔍 Preview links for changed docs

• docs/reference/edot-collector/config/custom-data-streams.md
• docs/reference/edot-collector/config/index.md