Docs Organization

[shangyian]

The docs should cater to the following audiences:

1. Users who are trying to onboard onto DJ for data modeling. The docs here might want to talk about how to interact with the DJ UI or any of its supported clients.
2. Users who are trying to set up DJ for internal use at a company. This may include setting up a customized query service, materialization service etc.
3. Users who are trying to contribute to DJ development and want to run the server locally.

We can group 2 and 3 together into a set of Developer docs, since users who are trying to do 2 will typically need to go through the workflow for 3. The docs for 1 should be in its own section, with the assumption that an instance of DJ is already running and ready for the user to use.