[doc][feat] add docs for broker load balancer

[Anonymitaet]

Search before asking

• [X] I searched in the issues and found nothing similar.

What issue do you find in Pulsar docs?

Need to add docs for https://github.com/apache/pulsar/issues/16691

What is your suggestion?

See comments below

Any reference?

N/A

Are you willing to submit a PR?

• [X] I'm willing to submit a PR!

[Anonymitaet]

Doc architecture: https://xmind.app/m/4XBe9w

TO-DO tasks:

• [ ] Restructure the IA
  • Consolidate existing docs
  • Add missing contents
  • Make docs in a logical way
• [ ] Add conceptual topics
  • [x] 1st version - Done by 6/8
  • [ ] 2nd version - WIP (iterate based on suggestions from @Demogorgon314 and @heesung-sn)
  • @Demogorgon314 has finished his 2-round review on 6/14
  • [ ] Add reference topics
  • [ ] Add task topics

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