doc: Add a layout demonnstration template.

Draw rectangle with optional text placed by the layout

tests/examples/wibox/layout/template.lua

@@ -0,0 +1,62 @@
+local file_path, image_path, luacovpath = ...   
+
+-- Set the global shims
+-- luacheck: globals awesome client tag drawin screen
+awesome = require( "awesome" )
+client  = require( "client"  )
+tag     = require( "tag"     )
+drawin  = require( "drawin"  )
+screen  = require( "screen"  )
+
+-- Force luacheck to be silent about setting those as unused globals
+assert(awesome and client and tag)
+
+local wibox     = require( "wibox"         )
+local surface   = require( "gears.surface" )
+local color     = require( "gears.color"   )
+local beautiful = require( "beautiful"     )
+
+-- If luacov is available, use it. Else, do nothing.
+pcall(function()
+    require("luacov.runner")(luacovpath)
+end)
+
+-- Create a generic rectangle widget to show layout disposition
+local function generic_widget(text)
+    return {
+        {
+            {
+                draw = function(_, _, cr, width, height)
+                    cr:set_source(color(beautiful.bg_normal))
+                    cr:set_line_width(3)
+                    cr:rectangle(0, 0, width, height)
+                    cr:fill_preserve()
+                    cr:set_source(color(beautiful.border_color))
+                    cr:stroke()
+                end,
+                widget = wibox.widget.base.make_widget
+            },
+            text and {
+                align  = "center",
+                valign = "center",
+                text   = text,
+                widget = wibox.widget.textbox
+            } or nil,
+            widget = wibox.layout.stack
+        },
+        margins = 5,
+        widget  = wibox.container.margin,
+    }
+end
+
+-- Let the test request a size and file format
+local widget, w, h = loadfile(file_path)(generic_widget)
+
+-- Emulate the event loop for 10 iterations
+for _ = 1, 10 do
+    awesome:emit_signal("refresh")
+end
+
+-- Save to the output file
+local img = surface["widget_to_svg"](widget, image_path..".svg", w or 200, h or 30)
+img:finish()