docs: some twitter/commons cleanup; pingpong->hello

There were some mentions of twitter/commons that became pants
There were some "../.." that became ".."

Use examples/hello as sample code instead of examples/pingpong. Pingpong's a great program to show off twitter/commons; it's not so great for a simple Pants example

Testing Done:
Doc generation now has many fewer failed-include warnings, yay

Reviewed at https://rbcommons.com/s/twitter/r/191/