docs: improve forms usage with examples

docs/components/content/examples/CheckboxExample.vue

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+<script setup>
+const selected = ref(false)
+</script>
+
+<template>
+  <UCheckbox v-model="selected" name="notifications" label="Notifications" />
+</template>

docs/components/content/examples/InputExample.vue

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+<script setup>
+const value = ref('')
+</script>
+
+<template>
+  <UInput v-model="value" />
+</template>

docs/components/content/examples/RadioExample.vue

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+<script setup>
+const methods = [{
+  name: 'email',
+  value: 'email',
+  label: 'Email'
+}, {
+  name: 'sms',
+  value: 'sms',
+  label: 'Phone (SMS)'
+}, {
+  name: 'push',
+  value: 'push',
+  label: 'Push notification'
+}]
+
+const selected = ref('sms')
+</script>
+
+<template>
+  <div class="space-y-1">
+    <URadio v-for="method of methods" :key="method.name" v-model="selected" v-bind="method" />
+  </div>
+</template>

docs/components/content/examples/SelectExample.vue

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+<script setup>
+const countries = ['United States', 'Canada', 'Mexico']
+
+const country = ref(countries[0])
+</script>
+
+<template>
+  <USelect v-model="country" :options="countries" />
+</template>

docs/components/content/examples/TextareaExample.vue

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+<script setup>
+const value = ref('')
+</script>
+
+<template>
+  <UTextarea v-model="value" />
+</template>

docs/components/content/examples/ToggleExample.vue

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+<script setup>
+const selected = ref(false)
+</script>
+
+<template>
+  <UToggle v-model="selected" />
+</template>