Allow custom Headings for River and FAQ component about brand HOT 2 OPEN

Yes, for some context the reason we default to h3 is to help enforce content order, whereby River should only ever appear beneath a SectionIntro, which uses a h2. Appreciate in hindsight how limiting this can be, so I would propose that we resolve this by maintaining the default, but forwarding an override.

cc. @danielguillan @ashygee

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Thanks for the context. The way I see it we should enforce content order through the templates we build, not the components themselves, otherwise we create dependencies that don't always make sense. Example, SectionIntro doesn't exist yet and might not be required in each context.

To think about it differently, we can offer different "levels" of components with increased control, e.g

1. Heading and Text basic building blocks to do anything.
2. River and SectionIntro components consisting of the above, but free to use independently.
3. RiverSection, a "section component" (words are hard 😄) consisting of the above and enforcing a certain order.

Think Brick (Primitives) -> House (Components) -> Neighborhood (Sections) -> City (Page template).

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