Consolidate UI developer docs

[ckadner]

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/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Remove this outdated OLD_README.md: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

Where-ever the UI is brought up (Docker Compose or via npm start) add the environment variables to disable the UI cache during development:

export REACT_APP_TTL=0
export REACT_APP_CACHE_INTERVAL=0

[RRM123]

@ckadner I can start working on this issue

[ckadner]

Thanks @RRM123 for taking this up

[RRM123]

Submitted a PR that resolves most of the documentation issues outlined above.
The remaining tasks are these changes to the developer-guide.md:

1. build the UI docker image from local code, push the image, deploy to cluster
2. developer setup with docker compose
3. code contribution guidelines, linting, updating npm dependencies, creating PRs, etc