docs(1.2.x) update upgrading.md file for Kong 1.2
docs(1.2.x) update upgrading.md file for Kong 1.2

This change basically involves replacing most of the text on this
page with the relevant section in
https://github.com/Kong/kong/blob/master/UPGRADE.md.

There are two major deviations from that file:
* I changed several header levels
* The `Upgrade Path for Patch Releases` section does not exist in
  UPGRADE.md, but I have left it here.

Mashape/getkong.org,Mashape/getkong.org,Mashape/getkong.org