Update doc to indicate ExtraTree support

[hcho3]

Closes #3064.

Treelite supports ExtraTreeRegressor and ExtraTreeClassifier starting from version 1.0.0, so this is just a matter of exposing the capability to FIL.

Also add ExtraTreeRegressor / ExtraTreeClassifier to the FIL test matrix.

[dantegd]

lgtm

[dantegd]

rerun tests

[dantegd]

@gpucibot merge

[codecov-io]

Codecov Report

Merging #3635 (0de43fe) into branch-0.19 (14bd6c1) will increase coverage by 0.35%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.19    #3635      +/-   ##
===============================================
+ Coverage        80.72%   81.08%   +0.35%     
===============================================
  Files              228      228              
  Lines            17629    17877     +248     
===============================================
+ Hits             14231    14495     +264     
+ Misses            3398     3382      -16     

Flag	Coverage Δ
dask	45.29% <ø> (+0.33%)	⬆️
non-dask	73.42% <ø> (+0.47%)	⬆️

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

Impacted Files	Coverage Δ
python/cuml/fil/fil.pyx	91.83% <ø> (ø)
python/cuml/linear_model/linear_regression.pyx	88.23% <0.00%> (-3.53%)	⬇️
python/cuml/dask/solvers/cd.py	100.00% <0.00%> (ø)
python/cuml/common/numba_utils.py	0.00% <0.00%> (ø)
python/cuml/neighbors/__init__.py	100.00% <0.00%> (ø)
python/cuml/internals/global_settings.py	100.00% <0.00%> (ø)
python/cuml/dask/preprocessing/encoders.py	100.00% <0.00%> (ø)
python/cuml/metrics/_ranking.py	98.57% <0.00%> (+0.02%)	⬆️
python/cuml/preprocessing/encoders.py	95.10% <0.00%> (+0.02%)	⬆️
python/cuml/common/array_descriptor.py	98.24% <0.00%> (+0.03%)	⬆️
... and 38 more

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