Add a list of flakiness causes with labels

What does this MR do and why?

This is the first step in properly identifying the cause of flakiness for our tests.

Each flakiness cause has:

• a label
• a description
• a difficulty to reproduce
• a resolution (or potential resolutions)
• links to examples

I already created the labels.

Review app at https://main-ee-101543.docs.gitlab-review.app/ee/development/testing_guide/flaky_tests.html.

Related to #378019 (closed).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.