docs: align sandbox quickstart defaults

[koriyoshi2041]

HUMAN:

• A human has tested these changes.

AGENT:

Docs-only change. I checked the changed markdown for trailing whitespace and searched the quickstart docs to confirm the old defaults were removed.

---

Why

A reviewer on #1428 suggested a follow-up PR to avoid creating both projects and Projects host folders, and to avoid a fixed old Docker image tag in the quickstart.

Summary

• Use $HOME/Projects as the host-side default for the Docker sandbox project directory in Unix and Windows quickstarts.
• Use ghcr.io/openhands/agent-canvas:latest instead of the old fixed 1.0.0-rc.11 image tag.
• Keep the container mount path as /projects.

Issue Number

Follow-up from review discussion in #1428.

How to Test

• git diff --check
• rg -n '\$HOME/projects|Join-Path \$HOME "projects"|agent-canvas:1\.0\.0-rc\.11' README.md README.windows.md || true

Video/Screenshots

Not applicable; docs-only quickstart change.

Type

• Bug fix
• Feature
• Refactor
• Breaking change
• Docs / chore

Notes

Risk is low. This only changes documented host defaults and image tag; it does not change the container's internal /projects path or runtime code.

[vercel]

@koriyoshi2041 is attempting to deploy a commit to the openhands Team on Vercel.

A member of the Team first needs to authorize it.