fix: switching mintlify openapi back to copy (#182)

### TL;DR

Replace symlinked OpenAPI file in Mintlify directory with a direct copy and update build scripts to maintain both copies.  Mintlify's Github app errored out when trying to deploy using the openapi.yaml symlink.

### What changed?

- Removed the symlink from `mintlify/openapi.yaml` to `../openapi.yaml`
- Added a direct copy of the full OpenAPI schema in `mintlify/openapi.yaml`
- Updated npm scripts in `package.json` to copy the OpenAPI file to the Mintlify directory after building or linting

Author: pengying