### What problem were solved in this pull request?

Problem: close #229 
当前的词法分析语法分析非常简单，那当遇到比较复杂的词法分析语法分析题目，比如表达式，有些同学就难以入手

### What is changed and how it works?
增加一个支持四则运算的功能，关键字是calc，虽然与标准的SQL不同，但是可以作为学习表达式入门使用。
使用示例：
```
(base) build_debug $ ./bin/observer -f ../etc/observer.ini -P cli
Successfully load ../etc/observer.ini
miniob > calc 2*(2*(2+3));
2*(2*(2+3))
20

miniob > calc (1+2) * (2 * (20+ -(5*1)))
(1+2) * (2 * (20+ -(5*1)))
90

miniob > calc 1, 2, 3;
1 | 2 | 3
1 | 2 | 3

miniob > calc 1 + 2 - 3+4;
1 + 2 - 3+4
4

miniob > calc 2* 4;
2* 4
8

miniob > calc 1, 2, 3;
1 | 2 | 3
1 | 2 | 3

miniob > calc 1 + 2 - 3+4;
1 + 2 - 3+4
4

miniob > calc 2* 4;
2* 4
8
```

### What problem were solved in this pull request?

Problem:
当前生成的github pages不太整洁方便，可以优化一下

### What is changed and how it works?
改用mdbook生成文档，并整理文档目录，调整链接