Definition of done: performance and availability implications of new changes

What does this MR do?

I started extending performance guidelines to consider additional aspects when delivering a feature that are to be considered by backend and frontend engineers.

This also put an emphasis that feature should be performant by default.

This follows our strategy to focus on ~availability as well as Velocity: https://about.gitlab.com/handbook/engineering/#prioritizing-technical-decisions.

Feedback

Everyone, what do you think about that?

What other aspects are important?

Next steps

I plan that once this gets reviewed I hope that this becomes a part of checklist that should be "validated" by person reviewing the code.

Does this MR meet the acceptance criteria?

Conformity

• Documentation created/updated or follow-up review issue created
• Merge request performance guidelines