Modernize design guidelines

[kra-mo]

Some of the design documentation is quite out-of-date and could be made better.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

• [ ] Screenshots are outdated. Here for example.
• [ ] The iconography guidelines don't reflect what's happening in practice anymore, at least of app icons.
• [x] This seems to be a dead link. This one as well.

[danirabbit]

In the future please open separate reports for each issue. It makes it easier to assign and track. Grouped reports are harder to triage and target to projects, etc

[danirabbit]

Fixes one of the dead links: https://github.com/elementary/hig/pull/58

[danirabbit]

This one fixes the other dead link by updating guidance around IPC https://github.com/elementary/hig/pull/59