docs(toolset): add YAML serialization examples

[MukundaKatta]

Summary

• add a Toolset YAML example that shows multiple serialized tools in one Toolset
• show how the same serialized Toolset can be passed to an OpenAIChatGenerator
• add a matching Agent example so the docs cover both init-parameter paths mentioned in the issue

Closes #11163

Verification

• reviewed the serialized shape against Haystack Tool.to_dict() and Toolset.to_dict() output structure
• ran git diff --check

Notes

• I did not run the docs site locally in this environment.

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[julian-risch]

@MukundaKatta Is this a duplicate of #11166 ?

[MukundaKatta]

@julian-risch Yes — duplicate of #11166 (that one was opened first and has slightly more complete examples). Closing this one; let's continue the review on #11166. Thanks!