Huge Improvement of Documentation

[xiaoruiDong]

Motivation or Problem

The previous documentation is outdated, with an obsolete description of RDMC and missing API references to a few modules.

Description of Changes

1. Change the theme of documentation to the Pydata theme
2. Introduce a hierarchical structure to the API references pages (in .rst) in /docs
3. Add references to the conformer_generation module, mathlib module, and reaction module
4. Introduce "How to cite RDMC" and "Credits" pages.

Testing

The new documentation is tested by building sphinx documents locally.