Improve nestedness of SASS styles
Add styles to web folder
First fix to make empty lines have transparent borders
Switch line borders from bottom to top

Previously borders on lines were on the bottom of the current
one, and this change switches it to the top of the next one.
This is useful so that where a junction between a code line
and a blank line happens, we can use the + operator (next
immediate sibling) to turn on a missing line.

We can't do this the other way around as CSS has no previous
immediate sibling operator.

I've left the new line in as red for debugging purposes,
will remove that in another commit.
Add note that manual rendering isn't recommended
Swap debug colour for regular colour
Remove unnecesary top border on section blocks
Sassify the CSS
Restore central dividing line

Probably got lost in an old sassification
Add in dotted border for line numbering