planning: `openapi/jan.json` to `cortex.json`

[gabrielle-ong]

Followup task from #1456

Problem

• openapi/jan.json is currently used as the single source of truth for API
• QN: should it be separated to cortex.json?
• What are the implications on Devops for cortex.so?

Success Criteria

Tasklist

• [ ]

[namchuai]

I think it's simple naming, need to coordinate with https://cortex.so because its using openapi/jan.json as well.

[dan-menlo]

@namchuai We have moved https://cortex.so into the Cortex repo, to make it easier to handle changes like this.

• Let's go ahead to change it to openai/cortex.json
• As part of this, please make sure to update Cortex Docs site as well (verify locally)

[gabrielle-ong]

✅ Verified cortex docs site locally, marking as complete, thanks @namchuai