Add example code

[marcqualie]

In order to test our code and see examples of what our apps should look like, we can add an examples/ folder and then run rubocop against those files on CI, so we can enforce our rules are working when doing PRs.

[IlkhamGaysin]

won't it be testing rubocop itself? if we wanna examples we can take a look to ruby docs?

[marcqualie]

The examples are what our application code should look like, not what the rubocop config looks like.

e.g. we want to ensure when we change a rule, it's enforcing the correct behaviour on an example file, like a rails controller or model.