[refactor][doc] Refactor the information architecture of Schema topics

[momo-jun]

Motivation

Improve the doc set of the Schema chapter to provide Pulsar users with a more accurate, easy-to-read, and comprehensive information experience.

Fixes #18151

Modifications

1. Curate a new Overview topic to introduce WHAT/WHY/HOW about Schema.
2. Curate a new Get Started topic to introduce how to construct a schema.
3. Improve the IA inside the Understand Schema and Schema evolution and compatibility topics. -- Will evaluate whether it makes more sense to merge Schema evolution and compatibility into Understand Schema with more reviews.
4. Relocate content in Manage Schema into Admin Interfaces - Schemas (new) and Understand Schema" topics.
5. Move examples and references from Client Libraries - Java/C++ to a single source inside Get started.
6. Remove duplicate or redundant information.

Note that this PR is a quick change to the overall IA of security topics. The detailed content updates and improvements will be continued and reviewed in a follow-up PR.

Documentation

• [x] doc
• [ ] doc-required
• [ ] doc-not-needed
• [ ] doc-complete

[momo-jun]

@RobertIndie Thanks for your review and comments. I addressed some of them and will leave the others to the next follow-up PR. Because this PR focuses more on the information architecture changes, and some changes (except the WHAT/WHY in the Overview) are a copy&paste work among topics. A thorough content review will be done in the following week through Google docs to add/change more details and I will invite you as a reviewer :)

[momo-jun]

@congbobo184 @RobertIndie I've created a Google doc to start the review/revise process on the content and added Zike's comments there. We can collaborate to review/revise in detail there.