refactor: mark common t* and D* as public

Author: AayushSabharwal

docs/src/API/model_building.md

@@ -3,6 +3,29 @@
 This page lists functionality and utilities related to building hierarchical models. It is
 recommended to read the page on the [`System`](@ref System_type) before this.
 
+## Common definitions of `t` and `D`
+
+ModelingToolkit provides common definitions for the independent variable `t` (time) and the
+derivative with respect to it `D`.
+
+```@docs
+ModelingToolkit.t_nounits
+ModelingToolkit.D_nounits
+ModelingToolkit.t
+ModelingToolkit.D
+ModelingToolkit.t_unitful
+ModelingToolkit.D_unitful
+```
+
+Users are recommended to use the appropriate common definition in their models. The required
+definitions can be imported with convenient aliased names. For example:
+
+```julia
+using ModelingToolkit: t_nounits as t, D_nounits as D
+```
+
+Allows using `t` and `D` to refer to `t_nounits` and `D_nounits` respectively.
+
 ## Hierarchical model composition
 
 The `System` data structure can represent a tree-like hierarchy of systems for building models

src/ModelingToolkit.jl

@@ -337,6 +337,7 @@ export DynamicOptSolution
 @public similarity_transform, inputs, outputs, bound_inputs, unbound_inputs, bound_outputs
 @public unbound_outputs, is_bound
 @public AbstractSystem, CheckAll, CheckNone, CheckComponents, CheckUnits
+@public t, D, t_nounits, D_nounits, t_unitful, D_unitful
 
 for prop in [SYS_PROPS; [:continuous_events, :discrete_events]]
     getter = Symbol(:get_, prop)