[doc] update docs and fix broken urls

[hubutui]

This pr update docs from ..registry import -> from ..builder import (#3846) and fix broken urls in docs.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #3947 into master will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3947      +/-   ##
==========================================
+ Coverage   62.32%   62.46%   +0.13%     
==========================================
  Files         221      221              
  Lines       16312    16308       -4     
  Branches     2756     2758       +2     
==========================================
+ Hits        10166    10186      +20     
+ Misses       5651     5628      -23     
+ Partials      495      494       -1     

Flag	Coverage Δ
#unittests	62.46% <100.00%> (+0.13%)	⬆️

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Impacted Files	Coverage Δ
mmdet/models/dense_heads/paa_head.py	91.59% <ø> (+1.17%)	⬆️
...mdet/core/bbox/iou_calculators/iou2d_calculator.py	96.77% <100.00%> (+8.31%)	⬆️
mmdet/models/losses/iou_loss.py	37.71% <100.00%> (-5.07%)	⬇️
mmdet/models/dense_heads/yolact_head.py	87.78% <0.00%> (+3.40%)	⬆️
mmdet/models/detectors/yolact.py	88.88% <0.00%> (+12.96%)	⬆️

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