finetune parsing start of line by new companion IMarkdownCommentHelper
+ detect excess slashes after '///'
+ determine minimum amount of whitespace per markdown comment
+ preserve additional whitespace at beginning of line
+ detect code block from 4+ leading spaces
+ ensuree DocCommentParser and JavadocParser are in sync

Also:
+ avoid showing closing ']' of links
+ resolve one warning (redundant null check)
+ fix and extend compliance settings in RunCompletionParserTests
+ fix & extend test source in ../Converter_23/src/markdown/testBug228648
+ fix 1-off-bug in ASTConverterMarkdownTest.verifyPositions()
  + locally improve code structure (constant part outside the loop)

fixes https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2808