[FLINK-15908][docs] Add description of support 'pip install' to 1.9.x documents.

[WeiZhong94]

What is the purpose of the change

This pull request adds description of support 'pip install' to 1.9.x documents.

Brief change log

• adds description of support 'pip install' to python_table_api.md.
• adds description of support 'pip install' to python_table_api.zh.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, Yarn/Mesos, 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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[hequn8128]

Some minor comments.

[hequn8128]

For the version of 1.9 series, maybe it's better to specify the version, for example:

# install the latest 1.9 version of PyFlink
python -m pip install apache-flink==1.9.*

The command will download the latest 1.9 pyflink.
What do you think?

[hequn8128]

ditto

[hequn8128]

Add a link to Pypi

[WeiZhong94]

@hequn8128 Thanks for your review! I have updated this PR according to your comments.

[hequn8128]

Merging...