Tracking issue for improvement course documentation and code comments

[rebelice]

This issue aims to address the lack of course documentation and code comments for TinySQL. We will start from this step to improve the user experience.

The documentation layout

## Project
### Introduction

### Topic
#### Knowledge topic
#### Related code

### Exercises
### References

Chinese version

## Project
### 简介

### XX主题
#### 相关知识
#### 相关代码

### 练习
### 引用

The code comments layout

• We'd better use /* Project 1: Your Code Here */ to identify where the user needs to fill in.
• We'd better add detailed comments to help users, such as:

  func MyFunc(a int, b int) (int, error) {
  /*  What does `MyFunc` need to do?
   *  explain parameters:
   *      a int: ....;
   *      b int: ....;
   *   explain return value:
   *     int: ....;
   *     error: ....;
   *   `MyFunc` may need to follow the steps:
   *       1. xxxx;
   *       2. xxxx;
   *       .....
   *    Some hints that might be useful:
   *       - you may need some structs
   *       - you may need to add some members for struct A
   *       - you need to implement 'FuncA' first
   *       - Be aware of concurrency issues
   *       .....
   */ 
  }
  

TODO

• refine project 1 #113
• refine project 2 #115
• Improvement course documentation and code comments for project 3 #111
• refine project 4 #119
• Project 5

[pupillord]

I have completed the improvement of project 3 #111