[feat][doc] add use cases + features + benefits + types for broker load balancing

[Anonymitaet]

What change does this PR introduce?

This PR adds the following conceptual docs for broker load balancing (topics in the red box below)

• Use cases
• Features
• Benefits
• Types

Doc development plan - Read before review

For the current doc of Broker Load Balancer, we want to tackle the following challenges:

• Create new documentation to cover all aspects of the feature (especially for the new extensible load balancer).
• Fill in any content gaps that may exist.
• Consolidate existing documentation into the new IA, as it is currently scattered and disorganized.
• Organize the documentation in a cohesive manner, ensuring it flows logically.

Consequently, we’ve created the following content development plans:

• Broker Load Balancing | Information Architecture
  This document presents a high-level architecture overview, including how we address content gaps and reconcile existing documentation.

• Broker Load Balancing | Doc Outline
  This document delves into the specifics of the content development plan, providing purpose and context for each section.

• doc

Ackowledgements

@Demogorgon314 @heesung-sn @D-2-Ed many thanks for your technical guidance and great support!

[Anonymitaet]

Previews go as expected:

[Anonymitaet]

@Demogorgon314 @heesung-sn @D-2-Ed this doc has been reviewed by you in Broker Load Balancing | Features + Benefits + Types + Use Cases, can you take a quick look at this PR?

@momo-jun @Huanli-Meng could you please help review this PR?

Thank you all!

[Huanli-Meng]

only 2 minor comments.