Improve documentation
The documentation has grown over time and while very detailed in many places it is missing content in others and makes it difficult to find certain information. Another aspect that's been brought up multiple times is a better documentation of changes, see #140, #420 and #411.

This is an umbrella issue for everything related to restructuring and updating the documentation content. Below is a work in progress structure which is split into four sections; the Sketch app, the command line interface, plugin development and the file format.

Each section follows the same structure, introducing wider _Concepts_ and optionally providing _Guides_ for real world use cases which allows for a range of entry level to more advanced content.

### Sketch Developer
- Sketch app
  - Using the sketch:// URL scheme
- `sketchtool` CLI
  - Concepts
    - Versioning
    - Usage
  - Export
    - Pages
    - Artboards
    - Layers
    - Slices
    - Preview
  - Run plugins
  - List document structure
  - Show locations
  - Extract metadata
  - Dump file into JSON
- Plugin
  - Guides
    - Run Script within Sketch
    - Install a plugin
    - Remove a plugin
    - Create a Plugin
    - Debug a plugin
    - Publish a Plugin using skpm
    - Publish a Plugin manually
    - Update a Plugin
    - Handle an Action
    - Use an external text editor
    - Add a native user interface
    - Add a user interface using a webview
  - Concepts
    - Plugin Bundle
    - Sketch JavaScript Environment
    - Manifest
    - Sketch menu
    - Commands
    - Actions
    - API Versioning
  - JavaScript API
    - DOM
    - UI
    - Data
    - Settings and State Preservation
    - Actions
    - Changelog
  - Internal API
    - Disclaimer
    - CocoaScript
    - Sketch Headers
- File format
  - Concepts
    - Versioning
    - File Bundle
  - Sample file
- API Reference