document new lazy loading feature

[lekoala]

@GuySartorelli here are the docs ;-)

PR reference : silverstripe/silverstripe-admin#1309

[GuySartorelli]

Sorry it took me so long to look at this. A few requested changes here.

Also I have started the process of migrating docs to a new repository, so this PR should be closed and a new one should be created there (sorry about that, poor timing).

[GuySartorelli]

I don't think we use pure javascript event handlers anywhere else in the documentation.... can you please change this to a jquery (and entwine if possible) reference to match the rest of the docs?

[lekoala]

$(el).one('lazyload') should achieve the same results, but since this is expected to be used in third party modules, i don't see the issue not using jquery or entwine. Actually, that should be real goal: provide platform agnostic support so that it works with anything, no ?
I'm not even sure how that would work with entwine (i don't think it's possible to listen to a custom event only once ?)

[GuySartorelli]

I've been thinking about this - I think the main issue is that if you just drop this pure js into LeftAndMain.extra_requirements_javascript you won't even be able to find the element you want to add a listener to unless it's in an entwine onmatch (except for when you directly load the form the tab is in via URL). So we probably should add that context to the example. And by that point you're already in a jquery context anyway.

[GuySartorelli]

Closed in favor of silverstripe/developer-docs#427