Add how to draw boxes page

docs/how-to/draw-boxes-around-subfigures.md

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+# How to draw boxes around subfigures
+
+If you want to show that several elements in a `Figure` belong together, you can do this by placing them all in a container.
+The trick is to use a nested `GridLayout` for each group of objects and place a `Box` at the same position as this `GridLayout`.
+Then the `alignmode = Outside(some_padding)` ensures that objects with protrusions sticking out, like `Axis`, are fully contained within the enclosing boxes.
+
+\begin{examplefigure}
+```julia
+using CairoMakie
+CairoMakie.activate!() # hide
+
+f = Figure()
+
+g1 = GridLayout(f[1, 1], alignmode = Outside(15))
+g2 = GridLayout(f[1, 2], alignmode = Outside(15))
+box1 = Box(f[1, 1], cornerradius = 10, color = (:tomato, 0.5), strokecolor = :transparent)
+box2 = Box(f[1, 2], cornerradius = 10, color = (:teal, 0.5), strokecolor = :transparent)
+
+# move the boxes back so the Axis background polys are in front of them
+Makie.translate!(box1.blockscene, 0, 0, -100)
+Makie.translate!(box2.blockscene, 0, 0, -100)
+
+Axis(g1[1, 1], backgroundcolor = :white)
+Axis(g1[2, 1], backgroundcolor = :white)
+
+Axis(g2[1, 1], backgroundcolor = :white)
+Axis(g2[1, 2], backgroundcolor = :white)
+Axis(g2[2, 1:2], backgroundcolor = :white)
+
+Label(f[0, :], "Two boxes indicate groups of axes that belong together")
+
+f
+```
+\end{examplefigure}