Include screenshots in the quick start guide

### Problem description

The quick start guide is very text-heavy which makes it hard to consume.

### Expected behavior

Include some screenshots to provide more context to the user and improve his confidence during the creation of his first app.

**Create your first app**
* Show a screenshot of a terminal running the Tabris CLI with the output of `tabris init`.
* Maybe: provide a screenshot of a GUI file manager showing a tree of the directories and files of the generated app. Easier to assimilate than `/`-separated paths, takes more effort to maintain though.

**Run the app**
* Add a screenshot of a terminal window with the output of `tabris serve`.
* Add a screenshot of the URL tab of the developer app with the URL from the `tabris serve` output.
* Add a screenshot of an app with an opened developer drawer.