Add Dekaf UI to README.md

[visortelle]

Fixes #xyz

Main Issue: #xyz

PIP: #xyz

Motivation

Dekaf UI is a well maintained open-source alternative to Pulsar Manager.

Dekaf is already recommended as a preferred option in the docs on pulsar.apache.org

It is also in apache/pulsar-helm-chart.

For better visibility I want to add it to this README.md.

Modifications

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

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