Adding proper top warning & removing others

@kennethgillen was concerned that the 4.4 docs are not all marked clearly enough so I've put the text from the top of each page into a proper warning box and removed the repetitive ones as this will be on every page anyway.

cc @joshmoore @jburel @jrswedlow - I guess the decision on this is somewhat political - is it ok to have a red warning box on a version we are still supporting?