docs(Swagger): Firefox Accounts OAuth Server API documentation

[xlisachan]

To view documentation on Ecosystem Platform locally

• Pull down this branch
• Within docusaurus.config.js of the Ecosystem Platform repo, update the value of spec (ln 39) to http://localhost:9000/swagger.json
• yarn start Ecosystem Platform
• Navigate to http://localhost:3000/ecosystem-platform/api

Because

• we want to autogenerate our OAuth Server API documentation

This pull request

• moves OAuth Server API endpoints out of Miscellaneous and Oauth sections within Auth Server API Swagger documentation
• adds the rest of the sections (URL Structure, Request format, Response format, etc) of the Auth Server API documentation into the Swagger docs
• updates fxa-auth-server/docs/api.md to just link to API doc in Ecosystem Platform
• adds OAuth Server API Swagger documentation
• adds and updates descriptions
• updates fxa-auth-server/docs/oauth/api.md to just link to API doc in Ecosystem Platform
• adds PR notes from Ben

Reference

• original Auth Server API doc
• OAuth Server API doc

Issue that this pull request solves

Closes: #12962

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• My commit is GPG signed.

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