Rearrange table of contents in docs

[cburgdorf]

What was wrong?

The current TOC with the API section expanded looks like this:

1. I believe the "Testing" section would be better placed within the API section

2. I think the root level of the API section is beginning to look cluttered.

How was it fixed?

1. Made API a subdirectory and turned the existing api.rst into three docs:

• Endpoint
• Common
• Exceptions

2. Moved the Testing API docs under the API subdirectory, too

With this change, the TOC with API docs expanded looks like this:

To-Do

• Clean up commit history

• Add entry to the release notes

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