docs: add new doc pages, remove outdated pages

Author: AayushSabharwal

docs/pages.jl

@@ -27,9 +27,13 @@ pages = [
         "Advanced Examples" => Any["examples/tearing_parallelism.md",
             "examples/sparse_jacobians.md",
             "examples/perturbation.md"]],
-    "Basics" => Any["basics/AbstractSystem.md",
-        "basics/ContextualVariables.md",
-        "basics/Variable_metadata.md",
+    "API" => Any["API/System.md",
+        "API/variables.md",
+        "API/model_building.md",
+        "API/problems.md",
+        "API/codegen.md",
+        "API/PDESystem.md"],
+    "Basics" => Any[
         "basics/Composition.md",
         "basics/Events.md",
         "basics/Linearization.md",
@@ -40,14 +44,6 @@ pages = [
         "basics/DependencyGraphs.md",
         "basics/Precompilation.md",
         "basics/FAQ.md"],
-    "System Types" => Any["systems/ODESystem.md",
-        "systems/SDESystem.md",
-        "systems/JumpSystem.md",
-        "systems/NonlinearSystem.md",
-        "systems/OptimizationSystem.md",
-        "systems/PDESystem.md",
-        "systems/DiscreteSystem.md",
-        "systems/ImplicitDiscreteSystem.md"],
     "comparison.md",
     "internals.md"
 ]