add CairoMakie extension doc page

Author: ytdHuang

docs/make.jl

@@ -59,6 +59,7 @@ const PAGES = [
         "Two-time correlation functions" => "users_guide/two_time_corr_func.md",
         "Extensions" => [
             "users_guide/extensions/cuda.md",
+            "users_guide/extensions/cairomakie.md",
         ],
     ],
     "Tutorials" => [
@@ -82,7 +83,7 @@ makedocs(;
     modules = [
         QuantumToolbox, 
         Base.get_extension(QuantumToolbox, :QuantumToolboxCairoMakieExt),
-        ],
+    ],
     authors = "Alberto Mercurio and Yi-Te Huang",
     repo = Remotes.GitHub("qutip", "QuantumToolbox.jl"),
     sitename = "QuantumToolbox.jl",

docs/src/users_guide/extensions/cairomakie.md

@@ -0,0 +1,21 @@
+# [Extension for CairoMakie.jl](@id doc:CairoMakie)
+
+## Introduction
+
+This is an extension to support visualization (plotting functions) using [`CairoMakie.jl`](https://github.com/MakieOrg/Makie.jl/tree/master/CairoMakie) library.
+
+This extension will be automatically loaded if user imports both `QuantumToolbox.jl` and [`CairoMakie.jl`](https://github.com/MakieOrg/Makie.jl/tree/master/CairoMakie):
+
+```julia
+using QuantumToolbox
+using CairoMakie
+```
+
+To plot with [`CairoMakie.jl`](https://github.com/MakieOrg/Makie.jl/tree/master/CairoMakie) library, specify the keyword argument `library = Val(:CairoMakie)` for the plotting functions. The supported plotting functions are listed as follows:
+
+!!! warning "Beware of type-stability!"
+    If you want to keep type stability, it is recommended to use `Val(:CairoMakie)` instead of `:CairoMakie`. See [this link](https://docs.julialang.org/en/v1/manual/performance-tips/#man-performance-value-type) and the [related Section](@ref doc:Type-Stability) about type stability for more details.
+
+| **Plotting Function** | **Description** |
+|:----------------------|:----------------|
+| [`plot_wigner`](@ref) | Wigner function |