docs - Update definition of done with e2e tests requirement

CE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22655

What does this MR do?

Adds the requirement for writing e2e tests to the definition of done.

Related issues

Closes https://gitlab.com/gitlab-org/quality/team-tasks/issues/67

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards