[Docs] Add note on resizing `disk_size` if image is larger

[romilbhardwaj]

Closes #1233.

Tested:

• [x] Docs rendered locally

[Michaelvll]

I think it would be useful if SkyPilot can be smarter for this. For example, if the disk_size is not specified and the image id is specified, we can set the disk_size to be the same as the image size.

For getting aws image info: https://docs.aws.amazon.com/cli/latest/reference/ec2/describe-images.html

For getting gcp image info: https://cloud.google.com/sdk/gcloud/reference/compute/images/list

[romilbhardwaj]

Stale, closing. We should do a new PR for Zhanghao's suggestion above