docs(1.x): mark "Clean up" in "Your first agent" as optional

[AllardQuek]

Fixes kagent-dev/kagent#3072

Summary

• Rename the "Clean up" heading in "Your first agent" to "Optional: clean up", so it no longer reads as the final step.
• Move "Next steps" above it, so the page ends on the common path. The callout text is unchanged.
• Update the anchor in "Your first MCP tool" from #clean-up to #optional-clean-up.

The optional cascading-delete follow-up from the issue is not addressed here.

Test plan

• Build the docs site (Hugo 0.160.1, as in CI) and confirm the new heading and section order render
• Confirm the link in "Your first MCP tool" resolves to the renamed section (#optional-clean-up)