rename functions to align with concepts

I'm wondering if `build_lesson()` should be renamed `build_site()` so it matches with `reset_site()`? but I guess it would clash with `{pkgdown}` and `{blogdown}` function names? If we call `reset_site()` `reset_lesson()` users might be confused about what that would do (reset content of the lesson files?). It seems that have `build_*` and `reset_*` use the same suffix would be useful though... 