Non text contrast updates

[alastc]

A bumper update to the non-text contrast understanding document to tackle 5 issues.

Preview of understanding doc.

Changes

#551 - Section on disabled elements is a bit of an explainer, shouldn't be in understanding doc, cut out the explanation.

#713 - Adding some similar examples as those noted in the article.

#896 - Adding examples, use this thread for inspiration.

#903 - Re-structure headings in the understanding doc.

#917 - Add CSS color values to the graphical examples.

Closes #551 #713 #896 #903 #917

[alastc]

@awkawk did the last change make the header thing better?

[awkawk]

@alastc Well.... Maybe if the situations were:
Situation A: Color is used to identify user interface components or used to identify user interface component states
Situation B: Color is required to understand graphical content (no colon - this one is the only one that has one now)
Situation C - fold into A as it is about a control and is really a conforming alternate version technique