Improve Debug and profile with concrete and completed case

[tisonkun]

... continuing #15 (review)

Thanks for your update! The content looks good.

However, given that it is a "Get Started" chapter, I'm afraid that it assumes too much readers know about GDB. A newbie walks through this chapter may be only able to see "(dlv)" or setup a breakpoint.

I'll appreciate it if you can elaborate TiDB works through section with a complete example. For example, we run SELECT 1; from client and setup breakpoint at the end of parsing and planning, display the AST and plan generated.

Later, add a link to GDB doc for those who want to dive deeper.