add docs

Author: vyudu

docs/src/systems/DiscreteSystem.md

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+# DiscreteSystem
+
+## System Constructors
+
+```@docs
+DiscreteSystem
+```
+
+## Composition and Accessor Functions
+
+  - `get_eqs(sys)` or `equations(sys)`: The equations that define the discrete system.
+  - `get_unknowns(sys)` or `unknowns(sys)`: The set of unknowns in the discrete system.
+  - `get_ps(sys)` or `parameters(sys)`: The parameters of the discrete system.
+  - `get_iv(sys)`: The independent variable of the discrete system.
+  - `discrete_events(sys)`: The set of discrete events in the discrete system.
+
+## Transformations
+
+```@docs; canonical=false
+structural_simplify
+```
+
+## Problem Constructors
+
+```@docs; canonical=false
+DiscreteProblem(sys::DiscreteSystem, u0map, tspan)
+DiscreteFunction(sys::DiscreteSystem, u0map, tspan)
+```