docs: added reactivity best practice

[shairez]

What is it?

• Feature / enhancement
• Bug
• Docs / tests / types / typos

Description

Added another best practices that avoids subscribing inside the component$ function.

Checklist:

• My code follows the developer guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• Added new tests to cover the fix / functionality

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[gioboa]

Nice @shairez 👍