fixed borken links on index page, 2.3 work

I fixed the page generated when no file is specified (index.php is
loaded with no GET parameters) by changing the list of links. Previously
"/markdown" was getting placed between the identifier and "/btec". All
is good now, apart from 2.1, which will be converted to the standard
format at some point.

More work has also been done on 2.3, which is due tomorrow. I should
probably spend more time writing the 2500+ words I need to for that than nice
detailed commit messages that no-one will ever read.
completed 2.3 (-D2), added 19 images, more...

I completed 2.3 up until the D2 task. Right now it's midnight,
and the assignment is due tomorrow at 11:05 PM, so I'll finish tomorrow
in class and after college. I added 19 images as part of the guides
given in the pass-level (PX) tasks. The images already in 2.3 were all
renamed to accomodate the new arrivals.

I also improved readme and license handling in index.php, and the way
$markdown is defined and used. I'll also update the documentation next
commit as I have changed the nginx rewrite rules again.