[Docs] Add FAQ

[dreamerlin]

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[codecov]

Codecov Report

Merging #413 (47a9f2a) into master (4a4385a) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #413      +/-   ##
==========================================
+ Coverage   86.66%   86.68%   +0.01%     
==========================================
  Files         101      101              
  Lines        7185     7185              
  Branches     1154     1154              
==========================================
+ Hits         6227     6228       +1     
+ Misses        726      725       -1     
  Partials      232      232              

Flag	Coverage Δ
unittests	86.66% <ø> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
mmaction/datasets/pipelines/augmentations.py	94.69% <0.00%> (-0.28%)	⬇️
mmaction/core/evaluation/accuracy.py	94.28% <0.00%> (+1.42%)	⬆️

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[innerlee]

Need to add the page into the index.rst