Fix induction recursion

[lenghuang]

Restructured #17

[jacobneu]

Maybe this PR could be coordinated with #29? I agree that natural number induction is an easier way to introduce induction than tree induction, but we should still keep the tree content. In #29 , I rename inductionintuition.md to treeinduct.md, and put the intro to induction/recursion in recind.md. It looks like some of your natural number induction content would fit nicely in the recind.md I'm writing.

Also, worth noting: currently, inductionintuition.md does not appear in SUMMARY.md, and therefore does not appear in the left sidebar (so it effectively hasn't been visible). This is fixed in #29, but should be addressed either way.

[lenghuang]

@jacobneu: Maybe this PR could be coordinated with #29? .... This is fixed in #29, but should be addressed either way.

Yea that sounds good to me. I'll put a pause on this for now then and start working on other SML Help things. I guess you can take the aspects that you think may fit well with #29, then based off what you take, I'll adjust the new treeinduct.md accordingly? Because right now at least, I'm not 100% how my tree induction content would be all that helpful beyond the initial "recursive thinking" aspect?