Document Event Detail types

[seanpdoyle]

The various events dispatched by Turbo can embed references to Turbo specific classes like FetchRequest, FetchResponse, and FormSubmission.

This commit adds dedicated entries to the Drive page, then links to those entries from the Events page.

[seanpdoyle]

@afcapel could you review this change as well?

[flavio-b]

@seanpdoyle, in the part about turbo:submit-end, it says that fetchResponse will be null when success: false, but that's not the case:

success can be false and we still get a fetchResponse, which I think is done by requestFailedWithResponse.

It seems like the only time fetchResponse is not present is when fetch itself throws an error, in which case the fetchResponse property is not even set, so we get undefined.

Also, the error property is not present by default, so it's always undefined, except when there's a fetch error, of course. I think this part of the docs could include a note to clarify that the error property is unrelated to the server response and has to do with a failure to submit the request (ex.: no internet connection).