Add document for data subset aggregation and dataset merge

[wonjuleee]

Summary

Add Level-5 Data Subset Aggregation
Add Level-7 Dataset Merge

How to test

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added the description of my changes into CHANGELOG.​
• I have updated the documentation accordingly

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below).

# Copyright (C) 2023 Intel Corporation
#
# SPDX-License-Identifier: MIT

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[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.01 ⚠️

Comparison is base (6835e0a) 78.70% compared to head (88d3de0) 78.70%.

Additional details and impacted files

@@                Coverage Diff                 @@
##           releases/1.2.0     #960      +/-   ##
==================================================
- Coverage           78.70%   78.70%   -0.01%     
==================================================
  Files                 231      231              
  Lines               26420    26420              
  Branches             5258     5258              
==================================================
- Hits                20795    20794       -1     
  Misses               4401     4401              
- Partials             1224     1225       +1     

Flag	Coverage Δ
macos-11_Python-3.8	77.73% <ø> (+<0.01%)	⬆️
ubuntu-20.04_Python-3.8	78.68% <ø> (-0.01%)	⬇️
windows-2019_Python-3.8	78.58% <ø> (ø)

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see 1 file with indirect coverage changes

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[vinnamkim]

LGTM.