Consolidate UI developer docs

There are currently 2 documents that contain UI developer content:
* [dashboard/origin-mlx/README.md](https://github.com/machine-learning-exchange/mlx/blob/main/dashboard/origin-mlx/README.md)
* [https://github.com/machine-learning-exchange/mlx/blob/main/dashboard/origin-mlx/developer-guide.md](https://github.com/machine-learning-exchange/mlx/blob/main/dashboard/origin-mlx/developer-guide.md)

The `README.md` should focus on how to use the MLX API with increasing levels of complexity:
1. point to quickstart guide, show commands to bring up the stack with and without UI, point to existing cluster deployment instructions
2. start the UI using existing docker image `docker pull mlexchange/mlx-ui:...` and `docker run ...`
3. start the UI using local code with `npm start` and the MLX API either from Docker Compose or a remote cluster
4. point to UI `developer-guide.md`

The UI `developer-guide.md` should focus on development aspects:
1. UI code structure and design principles
2. build the UI docker image from local code, push the image, deploy to cluster
3. developer setup with docker compose
4. code contribution guidelines, linting, updating npm dependencies, creating PRs, etc

Remove this outdated `OLD_README.md`:
* [dashboard/origin-mlx/OLD_README.md](https://github.com/machine-learning-exchange/mlx/blob/main/dashboard/origin-mlx/OLD_README.md)

Where-ever the UI is brought up (Docker Compose or via `npm start`) add the environment variables to **disable the UI cache** during development:
```Bash
export REACT_APP_TTL=0
export REACT_APP_CACHE_INTERVAL=0
```