docs: add project concept and update docs for global store architecture

[Angelmmiguel]

Updates documentation to reflect Rover's new global store architecture where task metadata, workspaces, and logs are stored centrally in ~/.rover/ instead of the legacy .rover/ folder in each project.

Changes

• Added new Project concept documentation explaining what a project is, automatic registration, project ID generation, and environment detection
• Added new Global Store advanced guide documenting the ~/.rover/ directory structure, rover info command, and configuration files
• Updated Task concept to reflect new storage location (~/.rover/data/projects/<project-id>/tasks/) and iteration-based file structure
• Updated Workspace concept to point to global store paths
• Updated Workflow and Sandbox concepts with corrected links and added Project to Next Steps
• Added Permission Issues troubleshooting guide for common file permission problems
• Reordered concept sidebar: Project → Task → Sandbox → Workspace → Workflow

Notes

The iteration file structure was also updated to reflect that inputs.json, entrypoint.sh, and workflow.yml are now stored per-iteration in tasks/X/iterations/Y/ rather than at the task root level.

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