[Doc] Add documentation for LiDAR-based 3D detection

[Tai-Wang]

There are several different tasks supported in the MMDet3D. This PR starts with LiDAR-based 3D detection and aims at adding a clear documentation for users to quickly learn how to perform a specific task via this tool.

There will be mainly four parts in this section: LiDAR-Based 3D Detection, Vision-Based 3D Detection, Multi-modality 3D Detection, 3D Segmentation.

[codecov]

Codecov Report

Merging #642 (d039837) into master (b4a8068) will not change coverage.
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  Coverage   51.07%   51.07%           
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  Hits         7778     7778           
  Misses       6934     6934           
  Partials      518      518           

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unittests	51.07% <ø> (ø)

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