Fix links in triggers page

[josephjclark]

Here's what https://docs.openfn.org/documentation/build/triggers looks like right now:

The "Managing State" and "Keep a cursor" sections don't link anywhere and aren't on the page.

This was pointed out by a user over in OpenFn/adaptors#654 (comment)

Looking at the legacy docs it looks things have been moved around and these got left hanging.

So I've made a few changes:

• Link the cursor stuff to the job writing guide bit on cursors
• Link to some state management stuff for triggers (although to be fair I don't think it's a very useful link)
• Removed some bullet points which I didn't think were very helpful
• Used Proper Nouns instead of "quotes" for jargon like Webhook Event, which looks a little bit less like we're making up words
• Adjusted the page structure to be a bit more useful

[josephjclark]

@taylordowns2000 @aleksa-krolls maybe either of you could check and merge this please :)