Update crash course

docs/tut/crash-course/8-cleanup.md

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+# Cleanup
+
+Sometimes you may need to do some cleanup when destroying a component or after
+a side-effect from a source update. Vide provides a function `cleanup()` which
+is used to register a cleanup callback for the next time the reactive scope
+it is called in re-runs.
+
+```lua
+locla mount = vide.mount
+local source = vide.source
+local cleanup = vide.cleanup
+
+local function Timer()
+    local count = source(0)
+
+    local con = game:GetService("RunService").Heartbeat:Connect(function(dt)
+        count(count() + dt)
+    end)
+
+    cleanup(function()
+        con:Disconnect()
+    end)
+
+    return create "TextButton" {
+        Position = UDim2.fromOffset(300, 300),
+        Size = UDim2.fromOffset(200, 50),
+
+        Text = function()
+            return "seconds: " .. math.floor(count())
+        end,
+    }
+end
+
+local unmount = mount(Timer)
+
+unmount() -- all registered cleanups are ran, heartbeat connection stopped
+```
+
+In the above example, this allows us to disconnect the heartbeat connection
+when the timer component is destroyed, whether that is from unmounting the app
+or if it is dynamically created by a control-flow function, which will be
+covered next.
+
+On a related note: the reason why `mount()` is used to create your app, is so
+that any top-level components that need to be cleaned up, can be cleaned up
+when the app is later unmounted, since `mount()` runs in a reactive-scope to
+track `cleanup()` calls. Vide's entire reactive system is independent from the
+life-time of instances; instances are just a side-effect of the reactive system.