Make the docs a bit more user friendly [#70840170]

- I've tried to break this down into sections that explain the files a bit more explicitly
- possible TODO: Move the google auth flow stuff to it's own section
- renamed the `generate_credentials` file to `generate-ga-credentials.py` as it is specific to google analytics
Is it worth describing a bit about what the entry point is? It's a python package path. The ones we list below are from this package. However, there is no reason why you couldn't write your own collector in `mypackage.foo.bar`, install it in the same virtualenv as this package and then use that.