[FLINK-23750][docs][table] Add documentation for Window Top-N after Windowing TVF.

[beyond1920]

What is the purpose of the change

The pr adds documentation for Window Top-N after Windowing TVF.

Brief change log

Window-topn.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 3443fcb (Wed Aug 25 12:34:42 UTC 2021)

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CI report:

• 77edfcd Azure: SUCCESS

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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, thanks.

[wuchong]

Some links are broken and docs can't build:

➜  docs git:(pull/16983) ✗ ./build_docs.sh
Start building sites … 
WARN 2021/08/27 20:30:27 Expand shortcode is deprecated. Use 'details' instead.
ERROR 2021/08/27 20:30:28 [en] REF_NOT_FOUND: Ref "docs/dev/table/sql/queries/window-join": "/Users/wuchong/Workspace/Flink/flink/docs/content/docs/dev/table/sql/queries/window-topn.md:31:524": page not found
WARN 2021/08/27 20:30:30 Expand shortcode is deprecated. Use 'details' instead.
ERROR 2021/08/27 20:30:31 [zh] REF_NOT_FOUND: Ref "docs/dev/table/sql/queries/window-join": "/Users/wuchong/Workspace/Flink/flink/docs/content.zh/docs/dev/table/sql/queries/window-topn.md:31:524": page not found
Built in 5394 ms
Error: Error building site: logged 2 error(s)

Please preview the docs in local before submitting pull request.

[beyond1920]

@wuchong I've updated the doc style. About broken link, window-join link is broken because window join documents are not merged yet. They would be ok after pr is merged.

[wuchong]

LGTM.