docs: align agent-sandbox version with go.mod

[FengyuanYin]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Updates the agent-sandbox version in docs/getting-started.md from v0.1.1 to v0.4.6 for both installation and cleanup.

AgentCube currently depends on agent-sandbox v0.4.6, while the getting-started guide still installed v0.1.1. This version mismatch could cause sandbox creation to time out, especially when using the warm pool example, even though the sandbox Pods were running.

Which issue(s) this PR fixes:

None.

Special notes for your reviewer:

The version now matches the sigs.k8s.io/agent-sandbox v0.4.6 dependency declared in go.mod.

Does this PR introduce a user-facing change?:

Yes.

Updated the getting-started guide to install agent-sandbox v0.4.6, matching AgentCube's dependency and preventing sandbox creation timeouts caused by incompatible versions.

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Details

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.98%. Comparing base (3b19390) to head (219129c).
⚠️ Report is 36 commits behind head on main.
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #453      +/-   ##
==========================================
+ Coverage   58.41%   59.98%   +1.57%     
==========================================
  Files          36       36              
  Lines        3463     3589     +126     
==========================================
+ Hits         2023     2153     +130     
+ Misses       1231     1216      -15     
- Partials      209      220      +11     

Flag	Coverage Δ
unittests	59.98% <ø> (+1.57%)	⬆️

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[acsoto]

Thanks for contributing but we do not need this for now because #446 is working on upgrading