Slide 2.3 (structure templates) - Removed part on folderish doctypes

Slide 2.3 (structure templates):
Removed part on folderish doctypes (merged in doctype slide)
Added context
Slide 2.2.2 - corrected slides id

Slide 2.2.2 - corrected slides id to avoid duplicates for exercises
Refactoring of slide 2.3 (structure templates)

Refactoring of slide 2.3 (structure templates)
Changed name of slide 2.4 to avoid spaces
Corrected backlinks because of name change