docs: add Sandbox CLI reference

[spandanareddib]

Purpose / Description

Adds a dedicated documentation page for the observal registry sandbox command group. This command was previously undocumented.

Fixes

• Fixes Add docs/cli/sandbox.md #859

Approach

• Created docs/cli/sandbox.md based on CLI --help output.
• Linked the new page in docs/SUMMARY.md.
• Added the command to the table in docs/cli/README.md.

How Has This Been Tested?

• Verified internal markdown links resolve correctly.
• Checked that the command documentation matches the current CLI output from uv run python -m observal_cli.main registry sandbox --help.

Checklist

• You have a descriptive commit message with a short title.
• You have performed a self-review of your own code.
• UI changes: N/A (Documentation only).