Add XGBoost and GBM feature interactions to docs

XGBFI package was implemented in h2o-3 in https://h2oai.atlassian.net/browse/PUBDEV-7739 and it needs to get documented. Please add this paragraph after Disabling XGBoost paragraph before Examples paragraph:

*XGBoost Feature Interactions *

Ranks of features as well as feature interactions by various metrics implemented in [ XGBFI | link to https://github.com/Far0n/xgbfi ] style.

The Metrics:

• Gain: Total gain of each feature or feature interaction

• FScore: Amount of possible splits taken on a feature or feature interaction

• wFScore: Amount of possible splits taken on a feature or feature interaction weighted by the probability of the splits to take place

• Average wFScore: wFScore divided by FScore

• Average Gain: Gain divided by FScore

• Expected Gain: Total gain of each feature or feature interaction weighted by the probability to gather the gain

• Average Tree Index

• Average Tree Depth

Additional features:

• Leaf Statistics

• Split Value Histograms

Usage is illustrated in Examples section.

Also add to the end of py/R examples code these lines:

py:

R:

Also add this paragraph after GBM Tuning Guide paragraph before Examples paragraph:

*GBM Feature Interactions *

Ranks of features as well as feature interactions by various metrics implemented in [ XGBFI | link to ] style.

The Metrics:

• Gain: Total gain of each feature or feature interaction

• FScore: Amount of possible splits taken on a feature or feature interaction

• wFScore: Amount of possible splits taken on a feature or feature interaction weighted by the probability of the splits to take place

• Average wFScore: wFScore divided by FScore

• Average Gain: Gain divided by FScore

• Expected Gain: Total gain of each feature or feature interaction weighted by the probability to gather the gain

• Average Tree Index

• Average Tree Depth

Additional features:

• Leaf Statistics

• Split Value Histograms

Usage is illustrated in Examples section.

And also add to the end of py/R examples code these lines:

py:

R: