Add a "Getting Started" page & node documentation to the wiki

[Blade67]

Adding a "Getting Started" page to the wiki would make it much easier to understand how this addon works and how to set it up. The example repo helps, but it isn't the minimal setup one would want to integrate this into their project.

I suggest the following layout for the pages:

Getting started
  - Database
    - What is a database?
    - How to set it up
  - Item setup
    - What are items
    - How to create new items
    - Adding categories to items
  - UI setup
    - How does the UI work (with the inventory-system addon)
    - Setting up basic UI
    - Additional functionality

The documentation for the setup doesn't have to be extremely precise, but enough to get the basics done. In addition there should be complete documentation for each Node (Database, Item, etc.) that covers their properties, signals and what they extend from.

[scriptsengineer]

Yes, I believe that now that the module is more stable and with only additional features to enter, I intend to focus on putting a minimum amount of documentation in the project.