chore: reset hugegraph version to 1.2.0

[VGalaxies]

Purpose of the PR

Subtask of #2265 and #2375.

For the convenience of releasing HugeGraph version 1.2.0, this PR includes the following changes:

• Unified the version number (revision) to 1.2.0
• Copied the build artifacts to the project root for better user experience
• Added README (draft) in the three submodules

Main Changes

Verifying these changes

• Trivial rework / code cleanup without any test coverage. (No Need)
• Already covered by existing tests, such as (please modify tests here).
• Need tests and can be verified as follows:
  • CI

Does this PR potentially affect the following parts?

• Nope
• Dependencies (add/update license info)
• Modify configurations
• The public API
• Other affects (typed here)

Documentation Status

• Doc - TODO
• Doc - Done
• Doc - No Need

[VGalaxies]

@imbajin @simon824 PTAL~

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c997f35) 66.32% compared to head (2c3a038) 66.30%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2382      +/-   ##
============================================
- Coverage     66.32%   66.30%   -0.02%     
+ Complexity      830      828       -2     
============================================
  Files           510      510              
  Lines         42186    42186              
  Branches       5840     5840              
============================================
- Hits          27978    27973       -5     
- Misses        11445    11448       +3     
- Partials       2763     2765       +2     

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