Dark mode for documentation

[itsdawei]

Description

Programmers should be entitled to dark mode! Also added a plugin to show the last edited time of a page.

TODO

• [x] Add button to switch between dark mode

Status

• [x] I have read the guidelines in CONTRIBUTING.md
• [x] I have formatted my code using yapf
• [x] I have linted my code with pylint
• [x] I have tested my code by running pytest
• [x] I have added a one-line description of my change to the changelog in HISTORY.md
• [x] This PR is ready to go