Add Haskell link to developers-guide.md (#409)

This PR adds a link to the Haskell-WASM documentation in the Developer
Guide section of the WebAssembly website. The addition provides an
important resource for developers who may be interested in learning more
about using Haskell with WebAssembly.

@tomayac

getting-started/developers-guide.md

@@ -23,6 +23,7 @@ layout: getting-started
   - [Pascal](https://wiki.freepascal.org/WebAssembly/Compiler)
   - [Zig](https://ziglang.org/documentation/master/#WebAssembly)
   - [Grain](https://grain-lang.org/docs/)
+  - [Haskell](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/wasm.html)
   - Python (via [Pyodide](https://pyodide.org/en/stable/) or [Nuitka (py2wasm)](https://github.com/wasmerio/py2wasm))
 - Use the compiled WebAssembly...
   - [from JavaScript code](https://developer.mozilla.org/en-US/docs/WebAssembly/Loading_and_running)