Docs: Add ModelHub documentation

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[codecov]

Codecov Report

Merging #6591 (09e3485) into master (1cd0f46) will decrease coverage by 0.39%.
The diff coverage is 100.00%.

❗ Current head 09e3485 differs from pull request most recent head 081fd14. Consider uploading reports for the commit 081fd14 to get more accurate results

@@            Coverage Diff             @@
##           master    #6591      +/-   ##
==========================================
- Coverage   87.76%   87.37%   -0.39%     
==========================================
  Files         422      420       -2     
  Lines       22906    22855      -51     
==========================================
- Hits        20103    19970     -133     
- Misses       2803     2885      +82     

Impacted Files	Coverage Δ
torch_geometric/nn/__init__.py	100.00% <100.00%> (ø)
torch_geometric/nn/model_hub.py	100.00% <100.00%> (ø)
torch_geometric/nn/models/correct_and_smooth.py	23.25% <0.00%> (-76.75%)	⬇️
torch_geometric/nn/models/rect.py	47.05% <0.00%> (-52.95%)	⬇️
torch_geometric/home.py	57.14% <0.00%> (-42.86%)	⬇️
...ch_geometric/transforms/remove_training_classes.py	60.00% <0.00%> (-40.00%)	⬇️
torch_geometric/transforms/to_device.py	64.28% <0.00%> (-35.72%)	⬇️
torch_geometric/utils/negative_sampling.py	91.97% <0.00%> (-7.30%)	⬇️
torch_geometric/nn/conv/pna_conv.py	92.77% <0.00%> (-7.23%)	⬇️
torch_geometric/nn/aggr/scaler.py	94.00% <0.00%> (-4.00%)	⬇️
... and 80 more

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