Custom build tutorial

Long promised ...

I've included an amended version of the introduction tutorial, which should help with #2437. The references to the hosted builds will have to be changed if and when these move to openlayers.org.

Unfortunately, I happened to use heatmap-earthquakes example, which has a problem #2477. The tutorial will need changing to handle the changes in this PR.

I've not changed the index for the moment, as I have another 'exploring the api' tutorial coming up, and will probably suggest some restructuring when I submit that.
Yes.

I think we should have a specific tutorial for the compilation app + ol3 together. To me "custom build" means "custom ol3 build", so I won't even talk about compiling app + ol3 together in buildExamples.md.