[FLINK-31670][doc] Using v3.0.0-docs branch for es connector docs build.

[reswqa]

What is the purpose of the change

In the doc, It still use "flink-version" for flink-connector-elastiacsearch instead of the version in the external repository.

Brief change log

• Using v3.0.0-docs branch for es connector docs build.

Verifying this change

Manually test.

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

[flinkbot]

CI report:

• a1679e3 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

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

[TanYuxin-tyx]

Thanks @reswqa, LGTM, I have tested on the local Mac and the change worked.

[reswqa]

Thanks @TanYuxin-tyx for the review, merged.