Add openapi-backend, openapi-client-axios to implementations

[anttiviljami]

Hi! 👋

We've been working on a couple of core implementations in the JavaScript world.

• openapi-backend, a framework agnostic backend library that handles routing, validation and mocking
• openapi-client-axios, an openapi client with a simple API for both frontend and backend. Fully supporting TypeScript workflows.

I'd be super cool if these could be mentioned on the implementations doc.

Thanks!

[MikeRalphson]

LGTM, thanks.