[Enhance] Add doc for ScanNet semantic segmentation data

[Wuziyi616]

Follow #664

[codecov]

Codecov Report

Merging #743 (9849cc6) into master (aae48a9) will not change coverage.
The diff coverage is n/a.

❗ Current head 9849cc6 differs from pull request most recent head ca5427e. Consider uploading reports for the commit ca5427e to get more accurate results

@@           Coverage Diff           @@
##           master     #743   +/-   ##
=======================================
  Coverage   48.94%   48.94%           
=======================================
  Files         208      208           
  Lines       15884    15884           
  Branches     2538     2538           
=======================================
  Hits         7774     7774           
  Misses       7608     7608           
  Partials      502      502           

Flag	Coverage Δ
unittests	48.94% <ø> (ø)

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