docs: add a basic example Lua plugin

docs/example-lua-plugin/main.lua

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+-- This message will be printed when the plugin is loaded
+print("Hi, this is an example Lua plugin")
+
+-- Setup a function called when the mailbox view is rendered
+koushin.on_render("mailbox.html", function(data)
+	print("The mailbox view for " .. data.Mailbox.Name .. " is being rendered")
+	-- Set extra data that can be accessed from the mailbox.html template
+	data.Extra.Example = "Hi from Lua"
+end)
+
+-- Wire up a new route
+koushin.set_route("GET", "/example", function(ctx)
+	ctx:String(200, "This is an example page.")
+end)
+
+-- Set a filter function that can be used from templates
+koushin.set_filter("example_and", function(a, b)
+    return a .. " and " .. b
+end)