Docs: Create a design-doc for how we could make the UI Extensions docs better

[leventebalogh]

Problems

• It's not clear from the docs what's a consumer and what is a provider
• It's not immediately clear from the docs what the framework is capable of doing
• The docs don't tell the reason why would you need an extension
• We don't have a clear and concise glossary (Example for Module Federation)
• We have no step-by-step guides (only best-practices and short examples)

Goals

• We have a design doc that has proposals for solving the issues above
• We have clear sub-issues in EPIC: Docs improvements for UI Extensions #1649 based on the accepted proposal in the design doc