View helpers page tidy up

This PR makes some improvements to the main view helpers page in the docs.

I've removed all of the content about individual view helpers from this page, since each of these helpers has its own page anyway, which this page already links to. This should make it easier to find things, since instead of landing on this one huge page, you'll end up on the page that just covers the helper you're looking for. This also makes the comments more useful, since they'll be focussed on the one topic.

I've added a note about custom view helpers that links to the "advanced usage" page (as personally I'd expect to find info about creating your own helpers on this page).

Thirdly I've added a link to the URL helper page which was missing. This does highlight the lack of docs we have for arguably the most important helper in the framework, but that's a problem for another day.