[Docs] Add docs and README for MinkUnet

[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 MinkUnet.

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.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Patch coverage: 54.68% and project coverage change: +0.63 🎉

Comparison is base (d84e081) 46.37% compared to head (f70abe0) 47.00%.

❗ Current head f70abe0 differs from pull request most recent head ff57ac8. Consider uploading reports for the commit ff57ac8 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2358      +/-   ##
===========================================
+ Coverage    46.37%   47.00%   +0.63%     
===========================================
  Files          259      275      +16     
  Lines        21390    22898    +1508     
  Branches      3379     3565     +186     
===========================================
+ Hits          9919    10763     +844     
- Misses       10827    11450     +623     
- Partials       644      685      +41     

Flag	Coverage Δ
unittests	47.00% <54.68%> (+0.63%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmdet3d/datasets/s3dis_dataset.py	78.08% <ø> (ø)
mmdet3d/datasets/scannet_dataset.py	82.75% <ø> (ø)
mmdet3d/datasets/transforms/__init__.py	100.00% <ø> (ø)
mmdet3d/datasets/waymo_dataset.py	17.72% <ø> (ø)
mmdet3d/evaluation/__init__.py	100.00% <ø> (ø)
...tion/functional/waymo_utils/prediction_to_waymo.py	0.00% <0.00%> (ø)
mmdet3d/evaluation/metrics/instance_seg_metric.py	94.44% <ø> (ø)
mmdet3d/evaluation/metrics/seg_metric.py	62.50% <0.00%> (ø)
mmdet3d/evaluation/metrics/waymo_metric.py	8.11% <0.00%> (ø)
mmdet3d/models/dense_heads/centerpoint_head.py	21.77% <0.00%> (ø)
... and 71 more

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