Add developer/API documentation

[brandomr]

On the developer side:

1. Define what is a codeset and describe how to create a new one
2. Define a toolset and describe how to create a new one
3. Describe how to add a subkernel
4. Describe how environments/julia is used in comparison to a codeset or subkernel
5. Describe how to add a custom kernel action (insofar as it isn't covered above)
6. Add existing architecture diagram from excalidraw to docs

On the API side:

1. Define and describe allowable requests and custom kernel actions
2. Provide example requests for various usages and example responses
3. Discuss how the Developer UI can be utilized for testing/extension/etc.