docs: document MongoDB mongodb+srv style DNS (horizon) endpoints

[ArnobKumarSaha]

Expands the bare mongodb+srv style DNS toggle mention in the MongoDB create-database guide into a full section, modeled on the Redis Announce Endpoints section.

• Explains the horizon mechanism (SRV-based discovery for external mongodb+srv clients).
• Documents prerequisites: DB TLS (upstream) and Expose via Gateway; Replicated Cluster mode only.
• Documents the SRV DNS Endpoints array input (one entry per member, count must match Replicaset Number).
• Links the new section from the mode table and the creation steps.

Note: references images/db-create/mongodb/horizon.png, which still needs to be added.

[ArnobKumarSaha]

Action needed before merge: this guide references docs/platform/guides/database-management/images/db-create/mongodb/horizon.png, which doesn't exist yet. A screenshot of the mongodb+srv style DNS toggle with the SRV DNS Endpoints inputs needs to be added at that path, otherwise the image will render broken.