Proposal: Augment dna.json to support clearer auto-generated api documentation

[neonphog]

Please see the changes to dna/dna.json:

• Addition of ReturnType to Functions items. Note holochain currently returns text/plain content-type for all calls (even if they return json data), hence everything for clutter marked as string.
• Addition of InputSchemaFile to Functions items that have a CallingType of json. This gives us more information about the expected parameters.
• Addition of OutputSchemaFile to Functions items that have a ReturnType of json. Unused at the moment (given holochain always returns text/plain) but can in the future give us an indication of output data format.
• Addition of Description to Functions items. This is a string (potentially markdown) that can give a more verbose indication of what functions do / how to use them.

Notes on accessing the api-docs:

• Works with the development server npm start... simply navigate to http://localhost:3000/clutter-api-docs.html
• Does NOT work with the production build, hcd/hcdev hosted ui site. Afaik we would need to eject the create-react-app environment to get the service-worker et all to stop overwriting the path with index.html. If anyone has another solution (ejecting adds a big configuration management overhead) would much appreciate it.
• You can also just navigate to the docs with file://, but you'll need some cors fixes: add cors headers to ui/webserver fn responses holochain-proto#741

[ddd-mtl]

To add to that I would also add the Description to Entries. And For Entries of type links, I would add more fields to describe the base, link and tags like I did in HoloChess.