rewrite the tutorial

update text for new UI, replace markdown cells with text cells, break it into subtutorials for each language.
now it loads in less than a second for me on my little laptop.
Fixes Issue #579 

remaining todo:
*i think the table with links to the two tuts for each language is good except maybe R is too hard to click on because it is just one letter.  how to make it larger?
* the table of contents of extra tuts on the right is too easy to miss.  consider folding it in to the main body or floating it left or something.
* the link to "scientific computing" is broken, filed an issue to fix it.
* the subtuts should still be greatly expanded!