docs: document parameter initialization in the tutorial

Author: AayushSabharwal

docs/src/tutorials/initialization.md

@@ -383,3 +383,56 @@ sol[α * x - β * x * y]
 ```@example init
 plot(sol)
 ```
+
+## Solving for parameters during initialization
+
+Sometimes, it is necessary to solve for a parameter during initialization. For example,
+given a spring-mass system we want to find the un-stretched length of the spring given
+that the initial condition of the system is its steady state.
+
+```@example init
+using ModelingToolkitStandardLibrary.Mechanical.TranslationalModelica: Fixed, Mass, Spring,
+                                                                       Force, Damper
+using ModelingToolkitStandardLibrary.Blocks: Constant
+
+@named mass = Mass(; m = 1.0, s = 1.0, v = 0.0, a = 0.0)
+@named fixed = Fixed(; s0 = 0.0)
+@named spring = Spring(; c = 2.0, s_rel0 = missing)
+@named gravity = Force()
+@named constant = Constant(; k = 9.81)
+@named damper = Damper(; d = 0.1)
+@mtkbuild sys = ODESystem(
+    [connect(fixed.flange, spring.flange_a), connect(spring.flange_b, mass.flange_a),
+        connect(mass.flange_a, gravity.flange), connect(constant.output, gravity.f),
+        connect(fixed.flange, damper.flange_a), connect(damper.flange_b, mass.flange_a)],
+    t;
+    systems = [fixed, spring, mass, gravity, constant, damper],
+    guesses = [spring.s_rel0 => 1.0])
+```
+
+Note that we explicitly provide `s_rel0 = missing` to the spring. Parameters are only
+solved for during initialization if their value (either default, or explicitly passed
+to the `ODEProblem` constructor) is `missing`. We also need to provide a guess for the
+parameter.
+
+If a parameter is not given a value of `missing`, and does not have a default or initial
+value, the `ODEProblem` constructor will throw an error. If the parameter _does_ have a
+value of `missing`, it must be given a guess.
+
+```@example init
+prob = ODEProblem(sys, [], (0.0, 1.0))
+prob.ps[spring.s_rel0]
+```
+
+Note that the value of the parameter in the problem is zero, similar to unknowns that
+are solved for during initialization.
+
+```@example init
+integ = init(prob)
+integ.ps[spring.s_rel0]
+```
+
+The un-stretched length of the spring is now correctly calculated. The same result can be
+achieved if `s_rel0 = missing` is omitted when constructing `spring`, and instead
+`spring.s_rel0 => missing` is passed to the `ODEProblem` constructor along with values
+of other parameters.