Add namespaces to code examples in docs

[GuySartorelli]

Currently most of the code examples in the docs do not include namespaces. This is a holdover from the SS3 days, and can in some cases result in a lack of clarity over whether namespaces should be included (e.g. most yaml config needs it but graphql for example doesn't always need namespaces).

We should add namespaces to all of the code examples so that it's clear how namespaces fit in with Silverstripe code.

[michalkleiner]

Also would be good to set a convention for the examples, e.g. for DOs you always do App\Model\ or Vendor\Module\Model\... and for controllers Vendor\Module\Controllers\... and/or without the Vendor if it's SS core code etc. Just for a bit of consistency so it's not all over the place.

[GuySartorelli]

That's a good idea. We can start adding that in this card but ultimately that sounds like a great consideration for #11 (I've added it to the list in that card)