Wiki: ODE Definitions

[B-rando1]

The ODE Definitions page mostly seems good. I remember reading it when I made the ODE prototype for Julia (#3931), and finding it helpful. I have a few notes:

• The PDController link is 404. I believe this is the correct link.
• There are a few instances of parenthesis that lack a space before them. E.g. first point of second heading has two of them.
• All the math is rendered in code blocks. Mathjax would look much nicer.

Contributes to #4401

[smiths]

@B-rando1, yes the link you supplied for the PDController is the correct link. That and your other suggestions sound good to me. 😄