Restructured JavaScript docs and new React samples

The JavaScript documentation has been re-structured to its own main menu item on docs.elmah.io, making room for more specific help articles: https://docs.elmah.io/logging-to-elmah-io-from-javascript/.

While moving around, we also updated the React documentation to reflect the recent changes happening in React and produced two new samples: elmahio-react and elmahio-react-typescript.