add webpack topic

[jrieken]

We should add a section about webpack, esp about migrating an extension towards webpack.

[octref]

Does #2403 #2254 cover it?

[DanTup]

Pine (@octref) The issue you linked is this one, did you mean something else? :)

[octref]

Oops, happens.

[jrieken]

Yeah, closing as dupe