fix(docs): Update the create a source plugin tutorial

[KyleAMathews]

Did an initial pass on improving the tutorial. Some notes on additional changes I think should be made:

• we really need to make the mental model clear for what a source plugin is. Having talked to a number of people in process of building a source plugin, they're often missing understanding of even very basic things. So similar to the main tutorial we should have people go step-by-step through the whole sweep of the data layer e.g. the different actions, showing them the LMDB file on disk, etc.
• the example needs to a lot simpler — Apollo, etc. gets in the way of understanding things. Plus we should steer people towards Got
• the tutorial mixes basic & advanced features — we should separate the basic tutorial from advanced features like relationships, images, content sync
• make really clear how node caching works i.e. that nodes stay in the db until they're deleted (or not touched). This comes up over and over.
• relatedly: make really clear how to do delta syncs. This also is rarely understood and comes up over and over

[DSchau]

Seems very sensible!