[Docs] Add docs and README for SPVCNN

[sunjiahao1999]

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Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.

Modification

Add docs and README for SPVCNN.

BC-breaking (Optional)

Does the modification introduce changes that break the back-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (b481efc) 47.00% compared to head (b0aa33f) 47.00%.

❗ Current head b0aa33f differs from pull request most recent head 972e523. Consider uploading reports for the commit 972e523 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           dev-1.x    #2372   +/-   ##
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  Coverage    47.00%   47.00%           
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  Files          275      275           
  Lines        22898    22898           
  Branches      3565     3565           
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  Hits         10763    10763           
  Misses       11450    11450           
  Partials       685      685           

Flag	Coverage Δ
unittests	47.00% <ø> (ø)

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[Tai-Wang]

Only need to revise the model/log links