Docs of all system of ODEs implemented

This PR contain docs of all system of ODEs implemented till now.
Added a test case in @XFAIL

A test case is failing in `./bin/test` due to matching failure. The issue for this is here(https://github.com/sympy/sympy/issues/7723) and discussion for this is here(https://github.com/sympy/sympy/pull/7678/commits)
Corrected small things
Updated comment
Merge pull request #7678 from KDMOE/sysode_docs

Sysode docs