[improve][doc] Information architecture redesigns and changes for Functions

[momo-jun]

Motivation

As part (Phase0 & Phase1) of the actions to improve the doc set of Pulsar Functions, the motivation is to provide Pulsar users with a more user-centric, accurate, and comprehensive information experience.

Modifications

1. Re-org topics (concept/task/reference) by applying topic-based authoring methodologies, which can contribute to revealing the complexity and granular pageview analytics.
2. Clarify tasks for different user personas (admin&ops/dev).
3. Introduce concepts about function instance, function worker, and window function in the Concepts topic
4. Add a new Quickstart topic to help beginners to get started with Pulsar Functions.
5. Refactor the content to make it easy to understand.
6. Fix outdated information, errors, and typos in original docs.
7. Streamline the visual styles of images and add several new illustrations.
8. Update cross-ref links in more topics due to topic ID changes.
9. Add a table to show the configurations that can be set via a YAML file --- implemented in PR: [improve][doc]single source of truth for functions CLI and add a table for yml configs #15389

To get a comprehensive view of the improved doc set (with a few screenshots for website preview) for Pulsar Functions, you can check out these Google docs:

• Overview and concepts: https://docs.google.com/document/d/1BautXTqWOxpKdNbhvGF2-KQtTYs4pRLvDPm5mLmb2ko/edit?usp=sharing
• Get started: https://docs.google.com/document/d/1dq8QtSjPsUFsxBnqwVqLNWLHjaNF0yBoLCLZt3EOYGY/edit?usp=sharing
• Admin tasks: https://docs.google.com/document/d/1gxrlXQITfbpO-18XIaB6T2CcKSGavOzFe1Fa0mY9klE/edit?usp=sharing
• Dev tasks: https://docs.google.com/document/d/1IKsBwmZco7ffPScVy05wg9qtVHwTYI7kHSA-5XU4oBA/edit?usp=sharing

The preview looks good. For more screenshots, see the above google docs.

Documentation

• doc

[github-actions]

@momo-jun:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)

[github-actions]

@momo-jun:Thanks for providing doc info!

[github-actions]

@momo-jun Please provide a correct documentation label for your PR.
Instructions see Pulsar Documentation Label Guide.

[momo-jun]

Hi @freeznet @nlu90, if you have reviewed the google docs, you can sign off the PR. Thank you.

[Anonymitaet]

@momo-jun some images are PNG while others are SVG. Do you want to keep consistent and use SVG only?

We talked about this before, SVG is better than PNG:

• Adaptive
  They adapt to the different sizes of the Chrome windows automatically.

• Sharper and smaller
  SVG images are more crisp and clean, and they are in a smaller size.

• Quality
  SVG is vector-based and can expand to any size without losing resolution

• Accessibility
  SVG is XML-based, and can be read by both human readers and machines like search engines

[momo-jun]

@momo-jun some images are PNG while others are SVG. Do you want to keep consistent and use SVG only?

We talked about this before, SVG is better than PNG:

• Adaptive
  They adapt to the different sizes of the Chrome windows automatically.
• Sharper and smaller
  SVG images are more crisp and clean, and they are in a smaller size.
• Quality
  SVG is vector-based and can expand to any size without losing resolution
• Accessibility
  SVG is XML-based, and can be read by both human readers and machines like search engines

Yes, I plan to revamp these PNG files in Phase2. They are reused from existing materials and not aligned with our style.