[feat][doc] add conceptual docs (bundle assignment/splitting/unloading...) for broker load balancing

[Anonymitaet]

What change does this PR introduce?

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

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]

For reviewers:

Here are some known issues of illustrations:

• They are not in a consistent style, some of them are not clear enough, and some images are missing. I tried to optimize them but failed. Previously I drew them in LucidChart, but we can't log into it suddenly. I've reported this issue and waiting for the response. I'll adjust these images once we have access to LucidChart.

• Some of them are too big. I've tried many solutions (e.g., https://linuxhint.com/add-images-markdown-modify-image-size/ and https://stackoverflow.com/questions/14675913/changing-image-size-in-markdown), but none of them worked. I guess this is a limitation of Docusarus since its official doc does not even mention "resizing image". I'll optimize them once I find a feasible solution.

[Anonymitaet]

For reviewers:

I write this on purpose. Reasons:

• Give users a positive expectation
• Call for a targeted list of early adopters interested in this new feature
• Improve SEO by adding relevant keywords

see TBD (the docs is WIP, stay tuned!).

[Anonymitaet]

@Demogorgon314 @heesung-sn @D-2-Ed this doc has been reviewed by you in Broker Load Balancing | Concepts, 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]

Some minor comments.