Improvements to dot-product and cross-product notes and slides

[bjmorgan]

The dot product and cross product are currently not well explained. This can be better explained by

1. describing these as "multiplications" of two vectors, but explaining that this is not simply multiplying lengths. We need to also account for relative orientations of two vectors.
2. Two standard "products": one that describes degree to which vectors are aligned, and another that describes the degree to which they are orthogonal.
3. Can show these graphically using trigonometric sin theta & cos theta definitions.
4. Can also show a graphical representation of how the different elements of the two vectors are multiplied and summed together (which should help the connection to the distance calculation in the first week 8 exercise).