### What changes were proposed in this pull request?
This PR adds the structured streaming UI introduction to the Web UI doc.
![image](https://user-images.githubusercontent.com/1452518/82642209-92b99380-9bdb-11ea-9a0d-cbb26040b0ef.png)
### Why are the changes needed?
The structured streaming web UI introduced before was missing from the Web UI documentation.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
N.A.
Closes#28609 from xccui/ss-ui-doc.
Authored-by: Xingcan Cui <xccui@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>