Added structure and subsection to Definition of Done

Added structure (i.e., subsections for backlog items, sprints and releases);
Added text for subsection about backlog items.
Correctd spellling error in file name

DefenitionOfDone.tex -> DefinitionOfDone.tex
Added text for Sprint section to DefinitionOfDone.tex
Added text for Release section to DefinitionOfDone.tex
Fixed some formatting errors in DefinitionOfDone.tex
Changed subsection title in DefinitonOfDone.tex

To be consistent with other subsection titles
Merge branch 'doc-definition-of-done'