[FLINK-29115] Improve python quickstart document

[yunfengzhou-hub]

What is the purpose of the change

This PR modifies Flink ML's python quickstart document to improve user experience and to align with flink's quickstart documents

Brief change log

• Improve python quickstart to align with pyflink quickstart document
• Change duplicate instructions on building or installing Flink ML to redirections to existing sections
• Fix grammar errors.

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)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)