Create a welcome page for debusine-server

After installing debusine-server the URL http://server shows a 404 message. Debusine-server can be used (the API works correctly, http://server/admin loads).

I think that would be nice to have a simple view in http://server saying that the instance of debusine-server is up and running as it is something that debusine admins or even users might expect to re-assure / confirm that the installation works.

Proposed text similar to the nginx:

<h1>Welcome to debusine-server!</h1>

If you see this page, the debusine-server is successfully installed and working.

Documentation available in ....

With a link to the online documentation?

In the future (after stage 1?) we could a variable in the settings with the admin name / contact or some other information.

@hertzog : let me know if this is something that should be done and if the text should be different. I would keep it as a Django view (to exercise Django code path) but with static information. Thanks!