Create OpenAPI docs

[kthchew]

Issue

Create more maintainable docs for the API, #46

Description

Adds a openapi-docs.yml to create nicer docs for the backend. This is intended to replace api/README.md (it's not very easy to work with Markdown tables!)

This currently doesn't entirely accurately reflect the actual behavior of the endpoints (e.g. most of them don't return different HTTP status codes based on the result; they probably should).

A process for actually building the docs (e.g. with GitHub Actions deploying to GitHub Pages) isn't done in this PR yet. There are VSCode extensions you can use to preview the docs, however (example).

Type Of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Test Steps

n/a

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Formatting

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