feat: Added new classification checkpoints

[frgfm]

This PR introduces the following modifications:

• adds 3 new checkpoints for resnet18, mobilenet_v3_small, rexnet1_0x
• fixed the header in docs

Any feedback is welcome!

[codecov]

Codecov Report

Merging #150 (31c7886) into master (754ebe5) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 31c7886 differs from pull request most recent head 00b43c7. Consider uploading reports for the commit 00b43c7 to get more accurate results

@@           Coverage Diff           @@
##           master     #150   +/-   ##
=======================================
  Coverage   89.16%   89.16%           
=======================================
  Files           8        8           
  Lines         203      203           
=======================================
  Hits          181      181           
  Misses         22       22           

Flag	Coverage Δ
unittests	89.16% <100.00%> (ø)

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Impacted Files	Coverage Δ
pyrovision/models/mobilenetv3.py	100.00% <100.00%> (ø)
pyrovision/models/resnet.py	100.00% <100.00%> (ø)
pyrovision/models/rexnet.py	100.00% <100.00%> (ø)

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