Add documentation for simulation throttling and limits

Author: fscottfoti

user_documentation/simulation_throttling.md

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+# Simulation Throttling and Limits
+
+To ensure fair usage and system stability, simulation requests are throttled based on both the number of simulations and the number of tiles processed.
+
+### Hourly Simulation Limits
+
+- **Default Limits:**  
+  - simulation per hour = 30  
+  - geoprocessings per hour = 10 
+  - quicksims per hour = 30
+- **Per-User Customization:**  
+  - Per-user customization can be done by MapCraft using simulationsHourlyLimit, geoprocessingHourlyLimit, and quicksimHourlyLimit
+- **Enforcement:**  
+  - The sum of simulations run in the past hour and the number of simulations in the current batch **must not exceed** the user's `simulationsHourlyLimit`.
+
+### Tile-Based Throttling
+
+- **Tile Counting:**  
+  - Each simulation counts the number of tiles in its input data.
+  - For throttling, a single simulation cannot count for more than 100 tiles, even if it processes more.
+- **Concurrent Tile Limits:**  
+  - **App-initiated simulations:** Up to 300 tiles can be processed concurrently.
+  - **API-initiated simulations:** Up to 600 tiles can be processed concurrently.
+  - **Per-user limit:** Each user can run up to 300 tiles concurrently.
+
+#### Example
+
+- If a project requires **150 tiles**, a user can run **3 simulations concurrently** (since each simulation is capped at 100 tiles for throttling).
+- If a project requires **50 tiles**, a user can run **6 simulations concurrently** (6 × 50 = 300 tiles).
+
+These rules ensure that no single user or project can monopolize system resources, maintaining performance and availability for all users.