[refactor][doc] Improve schema docs

[momo-jun]

Motivation

This is a follow-up PR to #18242 to improve the Schema docs, aiming to provide Pulsar users with a more accurate, easy-to-read, and comprehensive information experience.

Modifications

The content is revised and reviewed through Google docs.

1. Improve the description and illustrations of How Pulsar schema works.
2. Add an illustration to introduce Pulsar schema.
3. Incorporate schema evolution and subtopics into Understand schema.
4. Relocate native Avro to struct schema.
5. Add incremental changes to align with the code.
6. Add/correct descriptions, commands, and code examples, e.g., fixing [Doc] Java client examples for protobuf native schema #15035
7. Remove duplicate or redundant information.

Many thanks to @congbobo184, @liangyepianzhou and @RobertIndie for providing technical inputs and review comments.

//cc @Anonymitaet @DaveDuggins @D-2-Ed

Documentation

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

[labuladong]

Thanks for your work! Leave some style suggestions in the comments.

[momo-jun]

@labuladong thanks for your review. Comments have been addressed. PTAL.

@congbobo184 @liangyepianzhou @RobertIndie Do you have any other review comments?

[RobertIndie]

Thanks for your contribution! I left some comments here.

[momo-jun]

@RobertIndie @labuladong Thanks very much for your concrete review. Comments have been addressed. PTAL.

[RobertIndie]

LGTM. Thanks for your great work.

[labuladong]

Looks good to me~