docs: Fix Docker volume mount path for .openhands

[cbagwell]

The default location that OpenHands looks for .openhands directory has moved in recent versions but the docs were not completely updated.

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

The container can either mount at /root/.openhands or set an env var to tell location. I went with /root to match an existing document update that shows running openhands-cli inside a docker container and also mounts at /root.

I have seen some reports in Github Issues and Slack of users confused why conversations are no longer lasting past a container restart and other confusion on behavior differences in $HOME/.openhands after upgrading.