Sandboxes: drop the image parameter, fix /workspace paths in docs (0.3.0)

[ats3v]

Follows deepinfra/backend#4243, which removes image from the sandbox create API.

• Sandbox.create() / acreate() no longer accept image. The server ignores it now — it was an unvalidated field landing straight in the pod spec, and only images carrying /bin/bash could boot at all. Sandboxes always run the managed base image, which now ships curl, wget, git, jq, build-essential, node and npm. Sandbox.image still reports what a sandbox is running.
• Every doc example used /work/..., which the server rejects with a 500 — fs only accepts /workspace. Fixed in the module docstring, README, run_python docstring and examples/sandbox_quickstart.py, and the README now states that /workspace is the only path surviving stop()/start() or the idle auto-stop.
• 0.3.0, breaking for anyone passing image=.

Not published to PyPI yet.