LightGBM

[dask] add tutorial documentation (fixes #3814, fixes #3838) (#4030)

* [dask] add tutorial documentation (fixes #3814, fixes #3838)

* add notes on saving the model

* quick start examples

* add examples

* fix timeouts in examples

* remove notebook

* fill out prediction section

* table of contents

* add line back

* linting

* isort

* Apply suggestions from code review

Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

* Apply suggestions from code review

Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

* move examples under python-guide

* remove unused pickle import

Co-authored-by: Nikita Titov <nekit94-08@mail.ru>