[FLINK-23287][docs][table] Create user document for Window Join in SQL

[beyond1920]

What is the purpose of the change

The pr adds documentation for Window join.

Brief change log

Window-join.md

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (docs)

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Last check on commit f5a35f4 (Thu Aug 26 11:19:30 UTC 2021)

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[godfreyhe]

Thanks for the contribution @beyond1920 , LGTM overall

[godfreyhe]

cc @wuchong

[beyond1920]

@godfreyhe Thanks for review. I've updated, please have a look again.

[wuchong]

Thanks @beyond1920 , I left some comments.

[beyond1920]

@wuchong Thanks for review, I've updated. About Put it after "Join" page? It's strange to put it between "TopN" and "Window TopN"., could you please tell me how to move?

[beyond1920]

@wuchong Thanks for review, I've updated. Please have a look. Thanks a lot.

[beyond1920]

@wuchong Jark, I've updated the input schema to use a lower-case and non-reserved-keyword as the field names.

[wuchong]

Usually, we suggest to copy queries into SQL CLI and copy outputs to docs when writing examples.

[beyond1920]

@wuchong Thanks for suggestion, I've updated the results of sql query.

[wuchong]

LGTM.