Elaborate docs and create the first 7 lessons of the tutorials

[dodo920306]

This PR creates the first 7 lessons of the tutorials:

1. Introduction
2. Prerequisites
3. Server
4. Agent
5. User & Organization
6. Agent, Network, & Node
7. Channel & Chaincode

More importantly, this PR elaborate the docs pages by adding:

1. The dark mode
2. Navigation tabs
3. A logo picture
4. A "back-to-top" button
5. A "copy-code" button
6. Navigation footer
7. Admonitions
8. "Sans" fonts (better to look at)

Take the first page as the example:
Before:

After:

Additionally, the logo picture is added to the beginning of the README.

[dodo920306]

In order to emphasize the independence of the server and agent, I make them 2 seperate lessons with 2 different compose files.

[dodo920306]

You may want to build the docs and fix some expressions because I'm not a native English writer.

[dodo920306]

The 7th lesson "Lesson 6: Channel & Chaincode" will be the last lesson for this PR because any further feature that should be used in the following lessons is still under development. Event the Channel & Chaincode feature is effected by #733, which is also mentioned in the lesson.