[Documentation] Add documentation for Cypress test cases

[RyanL1997]

Issue Description

The functional tests of security-dashboards-plugin is currently based in the functional test repo. Even the FTR has the description of how to onboard test cases, we should also documents that in the README.md and DEVELOPER_GUIDE.md guide of security-dashboards-plugin repo itself.

Actions to Address this Issue

Document current Cypress test workflow with some instructions into README.md or DEVELOPER_GUIDE.md

Useful References

[1] The contributing file of functional test repo:
https://github.com/opensearch-project/opensearch-dashboards-functional-test/blob/main/CONTRIBUTING.md
[2] The test cases for plugins in functional test repo:
https://github.com/opensearch-project/opensearch-dashboards-functional-test/tree/main/cypress/integration/plugins

[RyanL1997]

[Triaged]