Add docs

docs/src/.vuepress/theme/components/SidebarGroup.vue

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+<template>
+  <section
+    class="sidebar-group"
+    :class="[
+      {
+        collapsable,
+        'is-sub-group': depth !== 0
+      },
+      `depth-${depth}`
+    ]"
+  >
+    <RouterLink
+      v-if="item.path"
+      class="sidebar-heading clickable"
+      :class="{
+        open,
+        'active': isActive($route, item.path)
+      }"
+      :to="item.path"
+      @click.native="$emit('toggle')"
+    >
+      <span>{{ item.title }}</span>
+      <span
+        v-if="collapsable"
+        class="arrow"
+        :class="open ? 'down' : 'right'"
+      />
+    </RouterLink>
+
+    <p
+      v-else
+      class="sidebar-heading"
+      :class="{ open }"
+      @click="$emit('toggle')"
+    >
+      <span>{{ item.title }}</span>
+      <span
+        v-if="collapsable"
+        class="arrow"
+        :class="open ? 'down' : 'right'"
+      />
+    </p>
+
+    <DropdownTransition>
+      <SidebarLinks
+        v-if="open || !collapsable"
+        class="sidebar-group-items"
+        :items="item.children"
+        :sidebar-depth="item.sidebarDepth"
+        :initial-open-group-index="item.initialOpenGroupIndex"
+        :depth="depth + 1"
+      />
+    </DropdownTransition>
+  </section>
+</template>
+
+<script>
+import { isActive } from '../util'
+import DropdownTransition from '@theme/components/DropdownTransition.vue'
+
+export default {
+  name: 'SidebarGroup',
+
+  components: {
+    DropdownTransition
+  },
+
+  props: [
+    'item',
+    'open',
+    'collapsable',
+    'depth'
+  ],
+
+  // ref: https://vuejs.org/v2/guide/components-edge-cases.html#Circular-References-Between-Components
+  beforeCreate () {
+    this.$options.components.SidebarLinks = require('@theme/components/SidebarLinks.vue').default
+  },
+
+  methods: { isActive }
+}
+</script>
+
+<style lang="stylus">
+.sidebar-group
+  .sidebar-group
+    padding-left 0.5em
+  &:not(.collapsable)
+    .sidebar-heading:not(.clickable)
+      cursor auto
+      color inherit
+  // refine styles of nested sidebar groups
+  &.is-sub-group
+    padding-left 0
+    & > .sidebar-heading
+      font-size 0.95em
+      line-height 1.4
+      font-weight normal
+      padding-left 2rem
+      &:not(.clickable)
+        opacity 0.5
+    & > .sidebar-group-items
+      padding-left 1rem
+      & > li > .sidebar-link
+        font-size: 0.95em;
+        border-left none
+  &.depth-2
+    & > .sidebar-heading
+      border-left none
+
+.sidebar-heading
+  color $textColor
+  transition color .15s ease
+  cursor pointer
+  font-size 1.1em
+  font-weight bold
+  // text-transform uppercase
+  padding 0.35rem 1.5rem 0.35rem 1.25rem
+  width 100%
+  box-sizing border-box
+  margin 0
+  border-left 0.25rem solid transparent
+  &.open, &:hover
+    color inherit
+  .arrow
+    position relative
+    top -0.12em
+    left 0.5em
+  &.clickable
+    &.active
+      font-weight 600
+      color $accentColor
+      border-left-color $accentColor
+    &:hover
+      color $accentColor
+
+.sidebar-group-items
+  transition height .1s ease-out
+  font-size 0.95em
+  overflow hidden
+</style>