Organize landing page (#1769)

The landing page (`README.md`) has been growing larger and less tidy over time as we try to cram more and more information into it.
This change makes a few edits to try to make the landing page shorter and more to the point:

* Streamline the opening lines and try to make them focus on the credibility of the system

* Break off the list of major features into its own subsection and try to highlight the ones that our current users say they benefit from the most

* Move a lot of the information about documentation, examples, Shader Playground, etc. into their own sub-pages to avoid clutter

* Break out the list of dependencies in the `License` section to make sure we are being accurate

With this change the landing page links to the User's Guide directly, so we probably need to get that rendering nicely ASAP.

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