Add Swagger/OpenAPI documentation for REST API

[kujoon226]

Description

This PR adds an initial Swagger/OpenAPI implementation for OED's REST API. It adds Swagger UI at /api-docs and documents the GET /api/conversions endpoint as an example. This is a prototype for maintainer feedback before documenting additional endpoints.

Partly Addresses #1695

Type of change

(Check the ones that apply by placing an "x" instead of the space in the [ ] so it becomes [x])

• Note merging this changes the database configuration.
• [x ] This change requires a documentation update

Checklist

(Note what you have done by placing an "x" instead of the space in the [ ] so it becomes [x]. It is hoped you do all of them.)

• [ x] I have followed the OED pull request ideas
• [ x] I have removed text in ( ) from the issue request
• [ x] You acknowledge that every person contributing to this work has signed the OED Contributing License Agreement and each author is listed in the Description section.

Limitations

Currently only the GET /api/conversions endpoint is documented as an example. Additional API endpoints still need Swagger/OpenAPI documentation. swagger-jsdoc is currently pinned to version 6.2.8 for compatibility with OED's current Node environment.