Modernize design guidelines

Some of the design documentation is quite out-of-date and could be made better. Some things in particular:

- [ ] Screenshots are outdated. [Here](https://docs.elementary.io/hig/widgets/creating-layouts) for example.
- [ ] The [iconography guidelines](https://docs.elementary.io/hig/reference/iconography) don't reflect what's happening in practice anymore, at least of app icons.
- [x] [This](https://docs.elementary.io/hig/widgets/selection-controls#:~:text=use%20a%20Mode%20Button%20instead.) seems to be a dead link. [This one](https://docs.elementary.io/hig/design-philosophy/concision#:~:text=other%20apps%20either-,through%20Contractor,-or%20over%20D) as well.