DOCSP-3182: add refrence to modeling manual pages

Author: skerschb

source/server/introduction.txt

@@ -26,6 +26,7 @@
 
    seealso:
      - :manual:`The MongoDB manual on Documents </core/document>`
+     - :manual:`The MongoDB manual on Data Modeling </core/data-modeling-introduction>` 
      - You can define the structure of your data for MongoDB using a JSON schema. See :manual:`Schema Validation </core/schema-validation>`.
      - Read more about what MongoDB can do, see :manual:`Introduction to MongoDB </introduction>`