docs: add e2e to writing tests doc

Description

Adds e2e section to writing-tests doc.

Follow up from #1291 (closed)

Related Issues

Resolves #1291 (closed)

How has this been tested?

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap