Major restructure of lessons

Moved pages, images, extra out of lessons/ to the base dir.

Moved all the topics to pages/ementa.html. I'll probably not make the text for
each topic anytime soon. Either way, it would be better to put the
lesson text with the practice activities.

Removed Ementa from home page.

Moved notebooks to lessons/. This will separate the grav and mag
notebooks and put them side-by-side with the practice texts.

No longer separate practice from lessons. Only Gravimetria, not
Gravimetria-praticas.