Refactor and test gleaner more thoroughly

[C-Loftus]

• removes all old code not being used (google docs, functionality to run nabu as a subcommand inside of gleaner, glcon, etc)
• gets rid of old documentation that is not relevant / outdates
• cleans up codebase
  • prefer propagating errors instead of fatal logging
  • make the cli easier to test
• adds e2e testing
  • makes all tests in golang instead of shell as well
  • use testcontainers