Add documentation: Overview of Frontend Testing

What does this MR do?

Adds documentation from the notes of the call the frontend team had where @iamphill and @Clement gave an overview to onboarding members of the team about the frontend testing current approach and strategy.

Are there points in the code the reviewer needs to double check?

Check if the documentation could be improved in any way.

Why was this MR needed?

New members sometimes lose some time finding out how to write proper tests from looking at the codebase. This guide aims at improving the immediate understanding of new contributors.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #45732 (closed)