Add V2 examples for SKLearn and XGBoost

[adriangonz]

What this PR does / why we need it:
Add examples and some docs around serving SKLearn and XGBoost models using the the KFServing V2 API.

Which issue(s) this PR fixes:

Fixes #2479

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

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[adriangonz]

/cc @cliveseldon @RafalSkolasinski

[seldondev]

Thu Oct 1 15:58:41 UTC 2020
The logs for [pr-build] [1] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2512/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2512 --build=1

[seldondev]

Thu Oct 1 15:58:49 UTC 2020
The logs for [lint] [2] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2512/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2512 --build=2

[axsaucedo]

Really cool, looks good
/approve

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

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