Color contrast rules [afw4f7, 09o5cg]: clarify behaviour on submit buttons

[Jym77]

Text has minimum contrast and Text has enhanced contrast are targeted at text nodes.

Some text appear as text on the page without being in a DOM text node, typically in a Submit button: <input type="submit" value="Next" /> has a text of Next and <input type="submit" /> has a text of (localised) Submit. The rules currently ignore these.

I'm not sure we should force the rules to look at these (they are complex enough, separate rules might be better at handling this). I think we should add an Inapplicable example to make the behaviour explicit.
(🤔 actually it might be better suited as a verbose example since it would be better as "inapplicable but bad" but then it would fail the requirements…)

[Jym77]

CG decision: we should try and come up with a definition of "text" to capture both, that way we can extend it later (select, …)