Add Design & Implementation Documentation

Each of these to be added as separate docs.
- [ ] Metapackage overview - concept and purpose
- [ ] Container bindings and sequencing
- [ ] Azure Functions testing helpers (including SpecFlow and XUnit specific helpers)
- [ ] Child object value retriever
- [ ] Teardown exception handling

For each item, cover:
- What problem does it solve
- How it solves the problem (and any aspects of the problem it specifically does not solve)
- Moving parts & extensibility
- Links out to "how-to" documents