[doc] add tutorial pages

[HerringtonDarkholme]

Basic:

1. simple pattern
2. simple kind
3. simple regex
4. multi-nodes match $$$
5. use fix

Intermediate:

1. Refine your pattern: pattern with context
2. regex in pattern
3. use and / any / not
4. use relational rule
5. refine relational rule I: field
6. refine relational rule II: stopBy
7. Example: extract child field using metavar & has

Advanced:

1. Refine your pattern II: understand how ast-grep matches
2. utility rule
3. utility rule II: recursion (use recursion to munch nodes)
4. manipulate metavar using transform
5. DasSurma trick to handle commas