[feat][doc] add Pulsar API content

[Anonymitaet]

What content does this PR add?

• This doc adds the content in green (1️⃣2️⃣3️⃣). They are totally newly created and in parallel with each other as independent topics in Pulsar docs.
• Currently, 2️⃣ is not placed in 4️⃣ (client libraries) because we’re optimizing the structure of 4️⃣, and 2️⃣ does not fit in 4️⃣ temporarily. Once we have a final and better solution, we’ll tweak the structure, but below is the structure that makes the most sense for now.
• This doc is the beginning of Building Great Developer Experience with API Content. More Pulsar API content will be created in the future.

Why add this content?

• Based on the user behavior metrics collected from GA, the Pulsar API doc is one of the top-viewed content. However, we do not have a page to explain Pulsar API systematically.
• This doc aims to provide landing pages (e.g. Get Started - Landing Page) for Pulsar API, Client API, and Admin API and organically correlate them. I make them​​ succinct, self-contained, and robustly linked rather than flooding users with too much info.

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This content has been reviewed and approved by @hangc0276 and @D-2-Ed, thank you!

And thanks for your suggestions! @BewareMyPower, @yuweisung, @tisonkun, @xiaogang-sn

cc @DaveDuggins

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• doc

[Anonymitaet]

Preview looks good:

• Pulsar API

• Client API

• Admin API

[momo-jun]

Nice work!