[ISSUE-48][FEATURE][FOLLOW UP] add docs for operator

[wangao1236]

What changes were proposed in this pull request?

For issue #48
I add docs about design and usage for operator .

Why are the changes needed?

Does this PR introduce any user-facing change?

How was this patch tested?

[codecov-commenter]

Codecov Report

Merging #261 (5737bd3) into master (fc6d49b) will decrease coverage by 0.41%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #261      +/-   ##
============================================
- Coverage     60.11%   59.70%   -0.42%     
- Complexity     1217     1377     +160     
============================================
  Files           150      166      +16     
  Lines          7602     8916    +1314     
  Branches        718      853     +135     
============================================
+ Hits           4570     5323     +753     
- Misses         2791     3318     +527     
- Partials        241      275      +34     

Impacted Files	Coverage Δ
...he/uniffle/client/impl/ShuffleWriteClientImpl.java	21.23% <0.00%> (-1.75%)	⬇️
.../org/apache/uniffle/server/ShuffleTaskManager.java	76.80% <0.00%> (-0.16%)	⬇️
...pache/uniffle/server/ShuffleServerGrpcService.java	0.87% <0.00%> (-0.03%)	⬇️
...e/uniffle/client/factory/ShuffleClientFactory.java	0.00% <0.00%> (ø)
...uniffle/storage/factory/ShuffleHandlerFactory.java	0.00% <0.00%> (ø)
...e/storage/handler/impl/LocalFileDeleteHandler.java	0.00% <0.00%> (ø)
...storage/handler/impl/HdfsShuffleDeleteHandler.java	0.00% <0.00%> (ø)
...n/java/org/apache/hadoop/mapreduce/MRIdHelper.java	0.00% <0.00%> (ø)
...mapreduce/task/reduce/RssInMemoryRemoteMerger.java	75.94% <0.00%> (ø)
...g/apache/hadoop/mapred/SortWriteBufferManager.java	79.89% <0.00%> (ø)
... and 21 more

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[zuston]

Great work!

[jerqi]

Should we put all the docs into the directory docs?

[jerqi]

Why do we delete the file docs/asset/rss_architecture.png

[wangao1236]

Why do we delete the file docs/asset/rss_architecture.png

Sorry, I may have deleted it by mistake, it has been restored

[jerqi]

LGTM