dpub aria role allowance for figure with figcaption

[scottaohara]

As discussed in https://github.com/w3c/dpub-aria/issues/42#issuecomment-1092558918, doc-example is a dpub variant of the figure role. This role being the same as figure but with the opportunity of being exposed as an "example" for its role description, needs to be an allowed role for the figure element.

ARIA in HTML (preview) is updating the allowed roles for figure with a child figcaption to account for this need from dpub aria.