work on new Pyret home page

[shriram]

It's in the pyret.org repo.

Work in the demo-home branch!

Remember to run npm install.

Run raco pollen start to build and display.

The index code uses Bootstrap CSS style entities.

Example programs are in the examples directory.

[shriram]

Consider having different example panes for each educator type, rather than just one global one. For instance, the binary tree may be intimidating to a high-school teacher but be very attractive to a college instructor.