[Docs] Add docs for the environment variables SkyPilot provides in one place

[Michaelvll]

Our environment variables are documented everywhere in the document, it would be good to have a centralized place as the YAML specification, Command Line Interface, etc.

The environments:
SKYPILOT_RUN_ID in https://skypilot.readthedocs.io/en/latest/examples/spot-jobs.html#an-end-to-end-example
SKY_NUM_GPUS_PER_NODE, SKY_NODE_RANK, SKY_NODE_IPS: in https://skypilot.readthedocs.io/en/latest/running-jobs/distributed-jobs.html

[Michaelvll]

This is still relevant. We should keep it open.

[github-actions]

This issue is stale because it has been open 120 days with no activity. Remove stale label or comment or this will be closed in 10 days.

[Michaelvll]

This has been fixed by #3715