Created by: vdepizzol
This PR adds a "Layout" page under the Foundations section.
It covers main principles behind designing consistent and responsive page layout, alongside essential global navigation details such as Context region.
I'm treating this page as just a starting point for more documentation on page composition to come. In future PRs we may break "Responsive design" and "Navigation" into its own page, and have specific component docs for areas such as PageHeader and SplitPageLayout.
Other sections I'm planning to include here in the future:
- Application examples
- Scrolling behavior