Docs: split the docstring reference into an API Reference tab

Move the Module/Types/Functions autodocs blocks and the documented-names
index off the Home page into docs/src/reference.md, with its own sidebar tab
after PyAMG. Home ends with a pointer to the reference page and the extension
pages. index.html shrinks from ~210KB to ~129KB.

Author: Sébastien Loisel

docs/make.jl

@@ -29,6 +29,7 @@ makedocs(;
         "JuMP" => "jump.md",
         "Gmsh" => "gmsh.md",
         "PyAMG" => "pyamg.md",
+        "API Reference" => "reference.md",
     ],
 )
 

docs/src/index.md

@@ -373,33 +373,9 @@ the coordinate-dedup default cannot produce.
   levels using purely geometric subdivision transfers (no AMG coarsening). Kept for
   cases that need geometric transfers specifically.
 
-# Module reference
+# Reference
 
-```@autodocs
-Modules = [MultiGridBarrier]
-Order   = [:module]
-Private = false
-```
-
-# Types reference
-
-```@autodocs
-Modules = [MultiGridBarrier]
-Order   = [:type]
-Private = false
-Filter = t -> !(nameof(t) in (:MGBModel, :Coef, :EpiPower, :deriv, :integral, :set_start, :mgb_solution, :solver_log, :On, :Broken, :Continuous, :Uniform, :gmsh_import, :amg_pyamg, :MGB3DFigure, :HTML5anim))
-```
-
-# Functions reference
-
-```@autodocs
-Modules = [MultiGridBarrier]
-Order   = [:function]
-Private = false
-Filter = t -> !(nameof(t) in (:MGBModel, :Coef, :EpiPower, :deriv, :integral, :set_start, :mgb_solution, :solver_log, :On, :Broken, :Continuous, :Uniform, :gmsh_import, :amg_pyamg, :MGB3DFigure, :HTML5anim))
-```
-
-# Index
-
-```@index
-```
+The full docstring reference (module, types, functions, and the index of all
+documented names) lives on the [API reference](reference.md) page; the
+extension APIs are documented on the [Plotting](plotting.md),
+[JuMP](jump.md), [Gmsh](gmsh.md), and [PyAMG](pyamg.md) pages.

docs/src/reference.md

@@ -0,0 +1,42 @@
+```@meta
+CurrentModule = MultiGridBarrier
+```
+
+# API reference
+
+The docstrings of the core module. The extension APIs are documented on their
+own pages — [Plotting](plotting.md) (`plot`, `MGB3DFigure`, `HTML5anim`),
+[JuMP](jump.md) (`MGBModel` and the modeling macros' helpers),
+[Gmsh](gmsh.md) (`gmsh_import`), and [PyAMG](pyamg.md) (`amg_pyamg`) — and the
+[index](@ref main-index) below links to everything.
+
+## Module
+
+```@autodocs
+Modules = [MultiGridBarrier]
+Order   = [:module]
+Private = false
+```
+
+## Types
+
+```@autodocs
+Modules = [MultiGridBarrier]
+Order   = [:type]
+Private = false
+Filter = t -> !(nameof(t) in (:MGBModel, :Coef, :EpiPower, :deriv, :integral, :set_start, :mgb_solution, :solver_log, :On, :Broken, :Continuous, :Uniform, :gmsh_import, :amg_pyamg, :MGB3DFigure, :HTML5anim))
+```
+
+## Functions
+
+```@autodocs
+Modules = [MultiGridBarrier]
+Order   = [:function]
+Private = false
+Filter = t -> !(nameof(t) in (:MGBModel, :Coef, :EpiPower, :deriv, :integral, :set_start, :mgb_solution, :solver_log, :On, :Broken, :Continuous, :Uniform, :gmsh_import, :amg_pyamg, :MGB3DFigure, :HTML5anim))
+```
+
+## [Index](@id main-index)
+
+```@index
+```