2021-06-11_1

Update theory part

Add TOC. Use (most) code examples from the talk, rm duplicate code
examples. Remove some ambiguous text parts. Remove comparison table (was
not very useful anyway, contained also numdifftools scipy which don't do
AD and are not talked about anywhere else). Refactor sections headings.
Give AD limitations their own section. All jax_autodiff_cookbook
reference. Switch on equation numbering.