[doc] Improve intro page
[doc] Improve intro page

To make the introduction easier to read, split the first section
into two, and explain first steps in a bullet list.

Update link to ebook.

Change-Id: Ie74f6aee291c3fc998a18a6a41829ccb318b9026

eclipsesource/tabris-js,eclipsesource/tabris-js,eclipsesource/tabris-js