Removed "Upgrading Guides" from homepage

- Dead link
- The "Upgrading Guides" tree was removed in:
  - 15f11a8e1f37f4823a5218989ca08b3bb9f2c026 on 3.0.x
  - 9982d9a019b1c3d1c4f8480b3b054617860d7ee7 on master