wip: add hydration question

src/data/question-groups/react/react.md

@@ -290,4 +290,10 @@ questions:
     topics:
       - 'Core'
       - 'Intermediate'
+  - question: What is Hydration?
+    answer: |
+      Hydration is the process of using client-side JavaScript to add interactivity to the markup generated by the server. When you use server-side rendering, the server returns a static HTML representation of the component tree. Once this reaches the browser, in order to make it interactive, React "hydrates" the static content, turning it into a fully interactive application.
+    topics:
+      - 'SSR'
+      - 'Intermediate'
 ---