Added master template and 404 error

Added a master template, which all other templates in the theme inherit from, and added a template for HTTP not found error.

- [x] finish the code
- [ ] gather feedback for my changes
- [x] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | none
| Doc           | https://github.com/sulu-cmf/docs/blob/master/detail-specification/000-foundation/DET-004-Routing.md