Use explicit form of Hamiltonian derivative in Kepler problem

[abhro]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• Any new documentation only uses public API

Additional context

Since the equations of motion are defined through the derivatives of the Hamiltonian in the code, RKNFirstDerivativeSentinel from using the DPRKN6 and ERKN4 solvers error because they get a dependence of the first derivative p from the position variable q (I think). Explicitly stating the equations of motion for ṗ and q̇ circumvents the problem and lets the documentation build, at the cost of making it a bit more obtuse. But I added the derivatives in the LaTeX block to help with that.
Also makes the imports explicit by just importing the used names because it was getting to be a pain to preface everything.