Update the docs (mostly spell checking)

doc/en/Authoring/Advanced_JSXGraph.md

doc/en/Authoring/Answer_Tests/Calculus.md

doc/en/Authoring/Answer_Tests/Equivalence.md

doc/en/Authoring/Answer_Tests/Rule_based.md

doc/en/Authoring/Answer_Tests/String.md

doc/en/Authoring/Answer_Tests/index.md

doc/en/Authoring/Authoring_quick_start.md

doc/en/Authoring/CASText.md

doc/en/Authoring/Equivalence_reasoning.md

doc/en/Authoring/Error_trapping.md

doc/en/Authoring/Errors.md

doc/en/Authoring/Future_proof.md

doc/en/Authoring/Inclusions.md

doc/en/Authoring/Inputs.md

doc/en/Authoring/JSXGraph.md

doc/en/Authoring/Multiple_choice_questions.md

doc/en/Authoring/Notes_about_performance.md

doc/en/Authoring/Parsons.md

doc/en/Authoring/Potential_response_trees.md

doc/en/Authoring/Question_blocks/Conditional_blocks.md