Improve the documentation landing page

[dwalkes]

The current Introduction page at https://oe4t.github.io/master/ doesn't give concise and clear steps in the form of a quick start for newcomers to the project.

To improve we should replace the current page with:

1. A path more focused on the end goal of understanding what OE4T is and isn't and how to build a functional first image.
2. Separate paths for different levels of experience, including

• Those who aren't familiar with Yocto/Openembedded but may have some experience with NVIDIA releases.
• Those who are familiar with openembedded/yocto but new to Tegra.

[dwalkes]

There's a new page we could potentially cross link at https://www.jetson-ai-lab.com/tutorials/yocto-on-jetson/

Cross linking https://docs.nvidia.com/jetson/archives/r39.2/DeveloperGuide/AR/YoctoOnJetson.html may also be good

[dwalkes]

We may also consider adding links to documentation from NVIDIA, such as https://developer.nvidia.com/embedded/downloads for the download center and https://docs.nvidia.com/jetson/ for the top level documentation pages