Refactor common issues in documentation

[ZwwWayne]

This PR split the common issues into a single page and add links in issue template.

[codecov]

Codecov Report

Merging #3530 into master will increase coverage by 0.22%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #3530      +/-   ##
==========================================
+ Coverage   58.74%   58.97%   +0.22%     
==========================================
  Files         205      205              
  Lines       13621    13696      +75     
  Branches     2310     2310              
==========================================
+ Hits         8002     8077      +75     
+ Misses       5244     5243       -1     
- Partials      375      376       +1     

Flag	Coverage Δ
#unittests	58.97% <66.66%> (+0.22%)	⬆️

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Impacted Files	Coverage Δ
mmdet/models/dense_heads/atss_head.py	23.24% <0.00%> (ø)
mmdet/models/dense_heads/corner_head.py	74.09% <100.00%> (ø)
mmdet/datasets/custom.py	35.71% <0.00%> (ø)
mmdet/models/losses/mse_loss.py	100.00% <0.00%> (ø)
mmdet/models/losses/smooth_l1_loss.py	100.00% <0.00%> (ø)
mmdet/models/backbones/resnet.py	99.30% <0.00%> (+0.01%)	⬆️
mmdet/models/backbones/regnet.py	95.61% <0.00%> (+0.03%)	⬆️
mmdet/models/roi_heads/mask_heads/maskiou_head.py	97.87% <0.00%> (+0.07%)	⬆️
mmdet/models/necks/fpn.py	93.82% <0.00%> (+0.07%)	⬆️
mmdet/core/fp16/decorators.py	96.77% <0.00%> (+0.10%)	⬆️
... and 24 more

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