Improve theme creation tutorial & process

[tabacitu]

Creating new themes has:

• docs in add docs on how to create a new backpack theme #447
• theme skeleton in https://github.com/Laravel-Backpack/theme-skeleton

This will provide 80% of the info needed to create a new theme. HOWEVER. We can still do A LOT more to improve how fast or easy it is to create a theme. We need to create a few themes, going through the tutorial, and improve the tutorial, improve the skeleton, improve the process. The faster we make this, the faster it will be for us to create new themes and for others to do the same.