design: Update the index and add some links
The design subdir README index was a little slim and
potentially out of date, and was missing hotlinks to
some documents that did exist.
Kata Containers design documentations, include the
-- Kata Containers architecture
+- [Kata Containers architecture](../architecture.md)
- Working scenarios of Kata Containers
-- API Design of Kata Containers
+- [API Design of Kata Containers](kata-api-design.md)
- Kata Containers lifecycle
+- [Design requirements for Kata Containers](kata-design-requirements.md)
