add tests to sphinx docs

[chongchonghe]

Description

Add a few pages to the Sphinx docs under the tests section.

Related issues

N/A

Checklist

Before this pull request can be reviewed, all of these tasks should be completed. Denote completed tasks with an x inside the square brackets [ ] in the Markdown source below:

• I have added a description (see above).
• I have added a link to any related issues see (see above).
• I have read the Contributing Guide.
• I have added tests for any new physics that this PR adds to the code.
• I have tested this PR on my local computer and all tests pass.
• I have manually triggered the GPU tests with the magic comment /azp run.
• I have requested a reviewer for this PR.

[BenWibking]

Minor comment about figures of horizontal lines. You can change those or not.

[chongchonghe]

Minor comment about figures of horizontal lines. You can change those or not.

Good point. I'll change it in a separate PR since it requires changing the code.