[plugin-architecture] Docs cleanup

[victorgcramos]

Currently we have lots of documented utilities and methods that are unnecessary, since they are self-descriptive.

We should avoid using our readme files for docs like that, because we may forget to update the docs, and hence create multiples sources of truth.

[victorgcramos]

Waiting for #2775

[victorgcramos]

We should add the docs about how to create apps/plugins and manipulate the politeiagui-scripts toolbox. (ref: #2827)

[victorgcramos]

Add docs or some user guideline describing the RFP process:

• What is an RFP proposal?
• What is an RFP submission?
• What is a Runoff Vote?
• How does the RFP process work?
• How to submit proposals to an RFP proposal?
• How to approve an RFP submission?
  ...

Or something like that.

[victorgcramos]

Add docs and guidelines for our core toolkit (#2869).

• What is a "slice service"?
• Why should I use the toolkit?
• How can I create a service?
• How to use services on apps?