[Doc] Add SUN RGB-D doc

[yezhen17]

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Motivation

Add SUN RGB-D doc.

Modification

• Add SUN RGB-D doc (mainly based on ScanNet doc).
• Modify ScanNet doc for consistency.

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.

[yezhen17]

TBD: Multi-modality related contents.

[codecov]

Codecov Report

Merging #770 (bc9eee0) into master (c07e960) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #770   +/-   ##
=======================================
  Coverage   49.03%   49.03%           
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  Files         210      210           
  Lines       15964    15964           
  Branches     2547     2547           
=======================================
  Hits         7828     7828           
  Misses       7634     7634           
  Partials      502      502           

Flag	Coverage Δ
unittests	49.03% <ø> (ø)

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[Wuziyi616]

We also need to add this doc to index.rst. And please add the corresponding Chinese doc (An empty file containing a head title is OK)

[Wuziyi616]

Almost ready for merge

[yezhen17]

Made a mistake in the dataset preparation part. @Wuziyi616 Please go through that part again. It's been too long that I've processed the raw data orz

[Wuziyi616]

Some minor typos in Chinese doc