Add ACP command

[neubig]

• 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

Now that OpenHands is added as the "openhands" command, this is a simpler and more robust way of running OpenHands.

[xingyaoww]

Probably let's keep it as is bc this will always use the latest version.

Later we should directly publish to Zed marketplace so user won't need to input this any more

[neubig]

Hmm but don't you think we should be consistent in how we recommend people use OpenHands? Here we recommend they use the openhands command: https://docs.openhands.dev/openhands/usage/run-openhands/cli-mode

Also, as an additional concern, if we always use the latest version there's a possibility that it'll suddenly break (hopefully not, but it's possible).

[xingyaoww]

Also, as an additional concern, if we always use the latest version there's a possibility that it'll suddenly break (hopefully not, but it's possible).

My primary concern now is that our ACP implementation is not stable yet, and we don't yet have a way to automatically check for ACP updates and force users to update -- I'm ok with changing the command but i'd hope we could do this in a few weeks when we are confident that ACP is a bit more stable