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
These lines seem to be [formatted as source](https://github.com/alphagov/performanceplatform-collector/blob/3rdpartyreadable/README.rst#configuration)