Document E2E tests

[zabeen]

Add a suite of E2E tests that can be run manually against a deployed Atlas instance to ensure it is working as expected.

[zabeen]

This branch on Atlas holds docs for E2E tests:
https://github.com/Anthony-Nolan/Atlas/tree/869-E2E-test-docs/EndToEndTestCases

The most critical search, repeat and donor tests have now been encoded as automated tests.
Docs also list the less critical tests for these features, as well as tests for other features.

• Not sure whether to keep the docs on Atlas, or move them to this new project?
• Also need to consider how the docs should be maintained going forwards. As not every scenario will be encoded as an automated test, it does make sense to maintain a written repo of manual tests, to be extended as required.