doc: optimize the guide for beginners and developers about pipcook

[yorkie]

How about this change: “How to use pipcook” > "How to use pipeline"?

[yorkie]

Currently the developer guide is not clear enough, it should include:

We need to make our developers map be clear as:

• Pipeline Developer(user)
• Plugin Developer
• Contributor

And in our original "Developer Guide", it means the documentation for the last contributor, it's a good start to refine our documentation TOC at this issue, too. The following is mine:

• Get Started
  • (MLApp)
  • Pipeline
  • Plugin
• Contribution Guide

[yorkie]

There is no doc / api reference for plugins themselves, so that the users are not clear what plugins we have and what parameters should be includes for specific plugin. We need to optimize plugin doc.

IMO we should split this into separate issues to address:

users are not clear what plugins we have.

It's good to maintain the plugins list so that user can browser there.

users are not clear what parameters should be included for specific plugin

To address this problem, that's @utkobe proposed improve the plugin API documentation. Moreover, because we are using TypeScript to write plugins, so we are absolutely able to provide more structured tools for better pipeline writings, for example:

• a tool to help generating plugin docs(https://github.com/rust-lang/docs.rs).
• hosted documentation of the maintained plugins at our project documentation.
• vscode plugin that makes suggestion for plugin parameters when writing the Pipeline.