Update documentation

[chaozg]

fix #470

Major changes I made:

• removed the manually inserted sections of Attributes and Methods from the docstring of class. Instead, we move relevant docstrings to the definition of respective properties and methods. Then we rely on sphinx to automatically capture the docstring.
• updated math equations, mainly in distribution and its subclasses, so the equations are rendered correctly.
• added experiment.mcmc to the overall API page, which will appear at the bottom of https://cuqi-dtu.github.io/CUQIpy/api/index.html

[chaozg]

Thank you @amal-ghamdi and I just included your suggestions in the last few commits, and am now re-requesting your and @jakobsj 's review.