[Doc] refine some documents

[ly015]

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Motivation

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

Review and refine the following documents:

• get_started.md
• contribution_guide.md
• faq.md
• migration.md
• switch_language.md

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduces changes that break the back-compatibility of this repo?
If so, please describe how it breaks the compatibility and how users should modify their codes 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 should be used to fix the potential lint issues.
2. The modification should be covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation should be modified accordingly, like docstring or example tutorials.

[ly015]

@Dai-Wenxun Could you please help update faq.md which contains outdated contents?

[codecov]

Codecov Report

Base: 76.00% // Head: 76.00% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (49c8a4e) compared to base (d131a87).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2058      +/-   ##
===========================================
- Coverage    76.00%   76.00%   -0.01%     
===========================================
  Files          137      137              
  Lines         9924     9918       -6     
  Branches      1627     1626       -1     
===========================================
- Hits          7543     7538       -5     
  Misses        2029     2029              
+ Partials       352      351       -1     

Flag	Coverage Δ
unittests	76.00% <ø> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
mmaction/models/recognizers/base.py	60.71% <0.00%> (-2.21%)	⬇️
mmaction/models/heads/base.py	77.64% <0.00%> (-0.26%)	⬇️
mmaction/datasets/transforms/formatting.py	83.49% <0.00%> (ø)
mmaction/models/heads/tpn_head.py	85.71% <0.00%> (+2.38%)	⬆️

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[ly015]

@cir7 Add docker part to get_started.md