docs: Update start.md

[ChloroplastYu]

What this PR does:

Add java-sdk uds demo doc

Which issue(s) this PR fixes:

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[ChloroplastYu]

/cc @wenxuwan

[wenxuwan]

/lgtm
/approve
/merge

@ChloroplastYu thanks for your contribution

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[LGTMNOTIFIER] This PR is LGTM

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