[Docs] Add not-found page extension.

[mzr1996]

Add not-found page and fix some links.

[codecov]

Codecov Report

Base: 0.02% // Head: 89.08% // Increases project coverage by +89.06% 🎉

Coverage data is based on head (bf0362d) compared to base (b8b31e9).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff              @@
##           dev-1.x    #1207       +/-   ##
============================================
+ Coverage     0.02%   89.08%   +89.06%     
============================================
  Files          121      143       +22     
  Lines         8217    10885     +2668     
  Branches      1368     1731      +363     
============================================
+ Hits             2     9697     +9695     
+ Misses        8215      947     -7268     
- Partials         0      241      +241     

Flag	Coverage Δ
unittests	89.08% <ø> (+89.06%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/apis/inference.py	0.00% <0.00%> (ø)
mmcls/datasets/transforms/compose.py
mmcls/models/heads/efficientformer_head.py	93.10% <0.00%> (ø)
mmcls/structures/utils.py	77.77% <0.00%> (ø)
mmcls/models/classifiers/timm.py	25.67% <0.00%> (ø)
mmcls/engine/hooks/retriever_hooks.py	72.72% <0.00%> (ø)
mmcls/models/backbones/mobilevit.py	91.15% <0.00%> (ø)
mmcls/models/backbones/swin_transformer_v2.py	89.63% <0.00%> (ø)
mmcls/models/retrievers/image2image.py	92.38% <0.00%> (ø)
mmcls/models/utils/layer_scale.py	86.66% <0.00%> (ø)
... and 133 more

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[tonysy]

LGTM