[DOCS] MigrationGuide - Adding note on Migrating Expectation Suites (#3959)
[DOCS] MigrationGuide - Adding note on Migrating Expectation Suites (#3959)

* Adding note on Expectation Suites and combining section on Validation operators
great-expectations/great_expectations,great-expectations/great_expectations,great-expectations/great_expectations,great-expectations/great_expectations