### What changes were proposed in this pull request?
Add a new `backpressure.svg` to replace the out-date one.
### Why are the changes needed?
After #1811, we refactor celeborn worker back-pressure logic, we should add new flowchart for user to understand.
### Does this PR introduce _any_ user-facing change?
Yes
### How was this patch tested?
Closes#1829 from zwangsheng/CELEBORN-905.
Authored-by: zwangsheng <2213335496@qq.com>
Signed-off-by: zky.zhoukeyong <zky.zhoukeyong@alibaba-inc.com>
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