Document usage in a local server

### Proposed change

Right now the documentation is tailored to jupyterhub usage. On the other hand, the adjustments needed to make nbgitpuller work with a local server are minimal.

- The landing page can state instead that nbgitpuller works with a local server too.
- The link generator can have a checkbox "not using jupyterhub" maybe even on by default, which removes `hub/user-redirect/` from the URL and optionally sets the default URL to `http://localhost:8888`

### Alternative options

Document why using nbgitpuller with a local server is a bad idea (if it is).

### Who would use this feature?

Instructors for courses without a jupyterhub.