[3.14] gh-137026: Add an explainer guide for asyncio (GH-137215) (#137581)

Co-authored-by: Alexander Nordin <[email protected]>
Co-authored-by: Peter Bierma <[email protected]>
Co-authored-by: 🇺🇦 Sviatoslav Sydorenko (Святослав Сидоренко) <[email protected]>
Co-authored-by: Carol Willing <[email protected]>
Co-authored-by: Kumar Aditya <[email protected]>
Co-authored-by: Stan Ulbrych <[email protected]>

Author: 7 people