### What changes were proposed in this pull request?
In this PR I'm adding Structured Streaming Web UI state information documentation.
### Why are the changes needed?
Missing documentation.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
```
cd docs/
SKIP_API=1 bundle exec jekyll build
```
Manual webpage check.
Closes#32433 from gaborgsomogyi/SPARK-35311.
Authored-by: Gabor Somogyi <gabor.g.somogyi@gmail.com>
Signed-off-by: Jungtaek Lim <kabhwan.opensource@gmail.com>