Update events listeners for page specific scripts

[amitaibu]

• Remove ihp:load
• Add comment about the recommend use of jQuery's on
• Add an example on how to avoid double binding.

[mpscholten]

This change is a BC break, not sure whether that's worth it. ihpLoadEvent is also used somewhere in the file, so we'd need to remove the usages as well in case we want to delete these variables

[amitaibu]

This change is a BC break, not sure whether that's worth it

In my tests I saw that ihp:load was not working as expected. So on the one hand it's BC break, but on the other - people using it might think it's working. Should we ask on Slack who's using ihp:load, see if we get people to answer?

[mpscholten]

I just checked and saw we're ourselves using this in several projects:

e.g. like this:

So let's keep this event here. But I'm fine with removing the outdated documentation on this and replacing it

[amitaibu]

hmm, I'm able to reproduce on my repo a case with $(document).on('ready turbolinks:load', function () { not working. So let's not merge yet.

[amitaibu]

Here's a reproducible example where the custom on ready handler isn't invoked transitioning into the page.

amitaibu/ihp-landing-page#7