docs: Add composables page

docs/src/guide/composables.md

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+# Composables
+
+## [useVueFlow](https://types.vueflow.dev/modules.html#useVueFlow)
+
+If you're using the options API of Vue you will soon notice that your access to the state of Vue Flow is limited.
+
+This is where the composition API comes in.
+
+The composition API and the power of provide/inject allows us to act more flexible with the way we provide states inside a component tree.
+Thus accessing the internal state of Vue Flow becomes super easy when using composition.
+
+```vue:no-line-numbers
+<script setup>
+import { useVueFlow, VueFlow } from '@braks/vue-flow'
+
+const { nodes, edges } = useVueFlow({
+  nodes: [
+    {
+      id: '1',
+      label: 'Node 1',
+      position: { x: 0, y: 0 },
+    }
+  ]
+})
+
+onMounted(() => {
+  console.log(nodes.value) // will log a single node
+  console.log(edges.value) // will log an empty array
+})
+</script>
+<template>
+  <VueFlow />
+</template>
+```
+
+`useVueFlow` exposes basically the whole internal state.
+The values are reactive, meaning changing the state values returned from `useVueFlow` will trigger changes in the graph.
+
+## [useZoomPanHelper](https://types.vueflow.dev/modules.html#useZoomPanHelper)
+
+Similar to the Vue Flow instance the zoom pan helper composable can be used to modify the viewport of the Vue Flow graph.
+It requires a valid Vue Flow store in its context.
+
+```vue:no-line-numbers
+<script setup>
+import { useZoomPanHelper } from '@braks/vue-flow'
+
+const { fitView } = useZoomPanHelper()
+</script>
+<template>
+  <button @click="fitView({ padding: 0.2, includeHiddenNodes: true })"></button>
+</template>
+```
+
+## [useHandle](https://types.vueflow.dev/modules.html#useHandle)
+
+Instead of using the Handle component you can use the useHandle composable to create your own custom nodes. `useHandle`
+provides you with a mouseDown- and click-handler functions that you can apply to the element you want to use as a node-handle.
+
+This is how the default handle component is built:
+
+```vue
+<script lang="ts" setup>
+import { useHandle, useVueFlow, Position, NodeId } from '@braks/vue-flow'
+import type { HandleProps } from '@braks/vue-flow'
+
+const { id, hooks, connectionStartHandle } = useVueFlow()
+const props = withDefaults(defineProps<HandleProps>(), {
+  type: 'source',
+  position: 'top' as Position,
+  connectable: true,
+})
+
+const nodeId = inject(NodeId, '')
+
+const { onMouseDown, onClick } = useHandle()
+const onMouseDownHandler = (event: MouseEvent) =>
+  onMouseDown(event, props.id ?? null, nodeId, props.type === 'target', props.isValidConnection, undefined, (connection) =>
+    hooks.value.connect.trigger(connection),
+  )
+const onClickHandler = (event: MouseEvent) => onClick(event, props.id ?? null, nodeId, props.type, props.isValidConnection)
+</script>
+<script lang="ts">
+export default {
+  name: 'Handle',
+}
+</script>
+<template>
+  <div
+    :data-handleid="props.id"
+    :data-nodeid="nodeId"
+    :data-handlepos="props.position"
+    :class="[
+      'vue-flow__handle',
+      `vue-flow__handle-${props.position}`,
+      `vue-flow__handle-${id}`,
+      'nodrag',
+      {
+        source: props.type !== 'target',
+        target: props.type === 'target',
+        connectable: props.connectable,
+        connecting:
+          connectionStartHandle?.nodeId === nodeId &&
+          connectionStartHandle?.handleId === props.id &&
+          connectionStartHandle?.type === props.type,
+      },
+    ]"
+    @mousedown="onMouseDownHandler"
+    @click="onClickHandler"
+  >
+    <slot :node-id="nodeId" v-bind="props"></slot>
+  </div>
+</template>
+```