Update Webhook Triggers: Add Async and Sync

[theroinaochieng]

This PR updates the webhook triggers documentation with async and synchronous modes.

This also updates the Webhook Trigger image.

Short Description

A one or two-sentence description of what this PR does.

Closes #<#736>

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[mtuchi]

maybe add ? at the end

[theroinaochieng]

I put this down as a statement more than a question Tuch. Do you think it'll make more sense as a question?

[mtuchi]

add ? at the end

[theroinaochieng]

I put this down as a statement more than a question Tuch. Do you think it'll make more sense as a question?

[mtuchi]

Ooh okay 🤔 , If the explanation is in a newline then it looks like a question but if it's oneline it's fine as is

**What this response represents:** It confirms that OpenFn has accepted the webhook and scheduled the workflow to run. It does **not** include the workflow’s output.

[mtuchi]

@theroinaochieng this looks good just tiny suggestion on questions like
when the response is sent: should end with ?

[taylordowns2000]

Hey @theroinaochieng , this is backwards. Async is the default mode (see app UI) and that means it replies immediately. Sync is the new, non-default option which will execute the workflow synchronously and reply when finished.