Write down the first version of Rule-based Optimization section

[tisonkun]

We don't have a file for it yet. May we create one on writing.

cc @yudongusa

[feitian124]

there are several blogs there such as https://pingcap.com/blog-cn/tidb-source-code-reading-7

[time-and-fate]

@tisonkun I'll write for this section. Please assign this to me~

[tisonkun]

@time-and-fate done. Thank you!