[hotfix][docs] fix doc for maven version and DataStream operators link URL

[showuon]

What is the purpose of the change

Fix documentation error.

1. We required Maven version 3.8.6.
2. DataStream operators in First step page is linking to an empty page. Fix it by directing users to DataStream operators overview page.

Brief change log

1. We required Maven version 3.8.6, not 3.
2. DataStream operators in First step page is linking to an empty page. Fix it by directing users to DataStream operators overview page.

Verifying this change

Doc change only.

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? (not applicable / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• 0eba1c64bf849bcdc1614f665cb206ff2e7a7a7c Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[showuon]

@reswqa , could you have a look when available? Thanks.

[showuon]

The CI test passed. Thanks.

[showuon]

@reswqa , kindly ping for another review. Thanks.

[showuon]

+1 for merging...

Thanks. :)