Documentation

[dfdx]

Just to track the most important parts that need to be documented:

• [x] derivative of an expression
• [x] derivative of a function
• [ ] gradient vs. Jacobian, vectorized vs. Einstein notation
• [ ] special functions
• [ ] diff rules (@scalardiff, @tensordiff, @specialdiff)
• [ ] code generation
• [ ] GPU
• [ ] benchmarks