Improve docs

docs/tut/crash-course/5-derived-state.md

@@ -1,58 +0,0 @@
-# Derived State
-
-You can create new state from existing states. This is known as *deriving
-state*.
-
-A function that wraps a state effectively becomes a state. If a state used
-inside a function is updated, the whole function can be re-ran to recompute
-its value.
-
-```lua
-local count = source(0)
-
-local function text()
-    return "count: " .. count()
-end
-
-create "TextLabel" {
-    Text = text
-}
-```
-
-Sometimes when using expensive computations to derive state, you only want to
-recalculate it once when a source state has changed
-
-If you wrap a source state with a regular function, its value will be recomputed
-every time you call that function.
-[`derive()`](../../api/reactivity-core.md#derive) accepts a functions whose
-return value will be cached, so that subsequent calls of this derived state
-will return the same cached value until one of its source states have changed.
-
-```lua
-local derive = vide.derive
-```
-
-```lua
-local count = source(0)
-
-local factorial = derive(function()
-    local n = 1
-    for i = 2, count() do
-        n *= i
-    end
-    return n
-end)
-```
-
-This can improve performance for expensive calculations.
-
-```lua
-create "TextLabel" {
-    Text = function()
-        return "factorial squared: " .. factorial() * factorial()
-    end
-}
-
-count(3) -- displays "factorial squared: 36"
-count(4) -- displays "factorial squared: 576"
-```