Add Definition of Done

documentation/definition_of_done.md

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+## Definition of Done
+
+- Cypress tests are passing and new tests have been added, if applicable
+
+- The code has been reviewed by another team member
+
+- The code has been merged to main branch
+
+- Functionality is manually tested both on local environment and on staging environment
+
+- The feature fulfills all the acceptance criteria defined in the corresponding user story