Update language extensibility guidance to recommend custom handlers

We should clarify whether custom handlers is the recommended route for language extensibility. Current most searches for language extensibility lands in [the wiki](https://github.com/Azure/azure-functions-host/wiki/Language-Extensibility). We need to update it to include custom handlers.

@pragnagopa @fabiocav Thoughts on how strongly we should recommend custom handlers? e.g., would it be something like “you can also use custom handlers” or “you really should be using custom handlers”?

Open the linked Language Extensibility wiki page and review its current guidance about language extensibility. Clarify how custom handlers should be presented and update the page with the agreed recommendation, from optional guidance to a stronger recommendation if maintainers decide that is appropriate.

Written by the indexing model from the issue text.