[fix][doc] Replace reference-configuration page with an external link

[SignorMercurio]

Motivation

The link Pulsar reference points to /reference, which is handled by Docusaurus as an SPA page and will result in 404. This is a known limitation of Docusaurus(facebook/docusaurus#3309) and the known workaround does not work well either.

Modifications

Therefore, we decided to use an external link in the sidebar to redirect users to /reference directly.

Verifying this change

• Make sure that the change passes the CI checks.

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

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 binary protocol
• The REST endpoints
• The admin CLI options
• Anything that affects deployment

Documentation

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• doc-not-needed
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• doc
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