move finished pages to root directory

Following the example of Gwern. I have a date field called "created",
and automatic archives page generation / index page generation are not
possible without modifying Hakyll source code
(see the [implementation](https://github.com/gwern/hakyll/blob/master/src/Hakyll/Web/Template/Context.hs#L214) of
getItemUTC)
Reform site.hs

The original one is more suitable for building blogs. After consulting
codes of gwern.net, I am removing chunks of unsuitable code to make it
more fit for static long-form sites.
Strengthen default template

This will be the only main template in the future.