Working through the docs with minor fixes and features added.

doc/en/Authoring/Deploying.md

doc/en/Authoring/Question_note.md

doc/en/Authoring/Quiz.md

doc/en/Authoring/Reviewing.md

doc/en/Authoring/Testing.md

doc/en/Authoring/index.md

doc/en/Developer/Development_track.md

doc/en/Developer/Docs.md

doc/en/Developer/Future_plans.md

doc/en/Students/Accessibility.md

doc/en/Students/Answer_assessment.md

lang/en/qtype_stack.php

questiontestrun.php

questiontype.php