Docs: update formidable icon to use registered symbol, update footer copy

[boygirl]

formidable icon should include ® , and footer copy should now read
"Formidable is a global design and engineering consultancy and open source software organization, specializing in React.js, React Native, GraphQL, Node.js, and the extended JavaScript ecosystem. We have locations in Seattle, London, Toronto, Denver, and Phoenix with remote consultants worldwide. For more information please visit formidable.com."

If it's faster, feel free to reference the analogous PR on victory-docs: https://github.com/FormidableLabs/victory/pull/1892/files