[improve][doc] Java example explaining code without using lombok

[AlvaroStream]

Motivation

Following strictly the example can make people that doesn't use lombok get this error:

No serializer found for class com.mytest.Product and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS)

Modifications

Increase the documentation on the code example.

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 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

I tried executing the sh start.sh:

[SUCCESS] Docusaurus website is running at: http://localhost:3000/

✔ Client
  Compiled successfully in 27.76s

client (webpack 5.75.0) compiled with 1 warning
<w> [webpack.cache.PackFileCacheStrategy] Skipped not serializable cache item 'Compilation/modules|/Users/arodriguez/Code/git_pulsar_alvaro/site2/.preview/pulsar-site/site2/website-next/node_modules/css-loader/dist/cjs.js??ruleSet[1].rules[6].use[1]!/Users/arodriguez/Code/git_pulsar_alvaro/site2/.preview/pulsar-site/site2/website-next/node_modules/postcss-loader/dist/cjs.js??ruleSet[1].rules[6].use[2]!/Users/arodriguez/Code/git_pulsar_alvaro/site2/.preview/pulsar-site/site2/website-next/src/css/custom.css': No serializer registered for Warning
<w> while serializing webpack/lib/cache/PackFileCacheStrategy.PackContentItems -> webpack/lib/NormalModule -> Array { 1 items } -> webpack/lib/ModuleWarning -> Warning

But I couldn't access the docs:

http://localhost:3000/docs/schema-understand/
http://localhost:3000/docs/2.10.x/schema-understand/

Was giving always Page Not found

Matching PR in forked repository

PR in forked repository: https://github.com/AlvaroStream/pulsar

[codelipenghui]

@AlvaroStream

I noticed you have removed

   @Builder
    @AllArgsConstructor
    @NoArgsConstructor

From the last comment #19019 (comment). We don't need to add the constructors if we have the annotation. The Lombok will generate them.

We should remove https://github.com/apache/pulsar/pull/19019/files#diff-fe3c06d8365d562427069af4376e699002169fdcf1047e836e771ce355756044R130-R134 if we have Lombok annotations.

[AlvaroStream]

@codelipenghui sorry, I missunderstood. Now is better?