Rework Get Started for accuracy

[stevemonaco]

Closes #413

The entire Test Drive section was revised for clarity, accuracy, and conciseness.

At some point, the Test Drive example was halfway switched to the core project + .Desktop project that Avalonia Template Studio creates. This PR updates the other bits left behind.

Removed one image because it was a duplicate IntelliSense tip.

choose-a-solution-template.md and install-the-avalonia-extension.md still need to be updated or removed. Most of getting-started.md was put into install.md as a way to check installation. None are in the sidebar currently.