docs: add child theme with custom homepage

closes #867

adds click-to-copy npm init @open-wc command to homepage, by creating a 
Vue child theme

docs/.vuepress/theme/components/Home.vue

@@ -0,0 +1,189 @@
+<style scoped>
+#homepage-npm-init-command {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding-top: 14px;
+  padding-bottom: 14px;
+  /* width: calc(100% - 28px); */
+  /* border-radius: 10px; */
+  color: white;
+  /* background: darkgrey; */
+}
+
+#homepage-npm-init-command {
+
+}
+
+</style>
+
+<template>
+  <main class="home" aria-labelledby="main-title">
+    <header class="hero">
+      <img
+        v-if="data.heroImage"
+        :src="$withBase(data.heroImage)"
+        :alt="data.heroAlt || 'hero'"
+      >
+
+      <h1 v-if="data.heroText !== null" id="main-title">{{ data.heroText || $title || 'Hello' }}</h1>
+
+      <p class="description">
+        {{ data.tagline || $description || 'Welcome to your VuePress site' }}
+      </p>
+
+      <clipboard-copy for="homepage-npm-init-command">
+        <code class="language-bash" id="homepage-npm-init-command">npm init @open-wc</code>
+      </clipboard-copy>
+
+      <p
+        class="action"
+        v-if="data.actionText && data.actionLink"
+      >
+        <NavLink
+          class="action-button"
+          :item="actionLink"
+        />
+      </p>
+    </header>
+
+    <div
+      class="features"
+      v-if="data.features && data.features.length"
+    >
+      <div
+        class="feature"
+        v-for="(feature, index) in data.features"
+        :key="index"
+      >
+        <h2>{{ feature.title }}</h2>
+        <p>{{ feature.details }}</p>
+      </div>
+    </div>
+
+    <Content class="theme-default-content custom"/>
+
+    <div
+      class="footer"
+      v-if="data.footer"
+    >
+      {{ data.footer }}
+    </div>
+  </main>
+</template>
+
+<script>
+import NavLink from '@theme/components/NavLink.vue';
+
+export default {
+  components: { NavLink },
+
+  beforeMount() {
+    import('@github/clipboard-copy-element');
+  },
+
+  computed: {
+    data () {
+      return this.$page.frontmatter
+    },
+
+    actionLink () {
+      return {
+        link: this.data.actionLink,
+        text: this.data.actionText
+      }
+    }
+  }
+}
+</script>
+
+<style lang="stylus">
+.home
+  padding $navbarHeight 2rem 0
+  max-width 960px
+  margin 0px auto
+  display block
+  .hero
+    text-align center
+    img
+      max-width: 100%
+      max-height 280px
+      display block
+      margin 3rem auto 1.5rem
+    h1
+      font-size 3rem
+    h1, .description, .action
+      margin 1.8rem auto
+    .description
+      max-width 35rem
+      font-size 1.6rem
+      line-height 1.3
+      color lighten($textColor, 40%)
+    .action-button
+      display inline-block
+      font-size 1.2rem
+      color #fff
+      background-color $accentColor
+      padding 0.8rem 1.6rem
+      border-radius 4px
+      transition background-color .1s ease
+      box-sizing border-box
+      border-bottom 1px solid darken($accentColor, 10%)
+      &:hover
+        background-color lighten($accentColor, 10%)
+  .features
+    border-top 1px solid $borderColor
+    padding 1.2rem 0
+    margin-top 2.5rem
+    display flex
+    flex-wrap wrap
+    align-items flex-start
+    align-content stretch
+    justify-content space-between
+  .feature
+    flex-grow 1
+    flex-basis 30%
+    max-width 30%
+    h2
+      font-size 1.4rem
+      font-weight 500
+      border-bottom none
+      padding-bottom 0
+      color lighten($textColor, 10%)
+    p
+      color lighten($textColor, 25%)
+  .footer
+    padding 2.5rem
+    border-top 1px solid $borderColor
+    text-align center
+    color lighten($textColor, 25%)
+
+@media (max-width: $MQMobile)
+  .home
+    .features
+      flex-direction column
+    .feature
+      max-width 100%
+      padding 0 2.5rem
+
+@media (max-width: $MQMobileNarrow)
+  .home
+    padding-left 1.5rem
+    padding-right 1.5rem
+    .hero
+      img
+        max-height 210px
+        margin 2rem auto 1.2rem
+      h1
+        font-size 2rem
+      h1, .description, .action
+        margin 1.2rem auto
+      .description
+        font-size 1.2rem
+      .action-button
+        font-size 1rem
+        padding 0.6rem 1.2rem
+    .feature
+      h2
+        font-size 1.25rem
+</style>

docs/.vuepress/theme/index.js

@@ -0,0 +1,3 @@
+module.exports = {
+  extend: '@vuepress/theme-default',
+};

package.json

@@ -34,6 +34,9 @@
     "vuepress:copy-static": "cp -f packages/demoing-storybook/demo/custom-elements.json docs/.vuepress/public/demoing/demo/custom-elements.json",
     "vuepress:start": "vuepress dev docs"
   },
+  "dependencies": {
+    "@github/clipboard-copy-element": "^1.1.1"
+  },
   "devDependencies": {
     "@commitlint/cli": "7.2.1",
     "@commitlint/config-conventional": "7.1.2",

yarn.lock

@@ -1295,6 +1295,11 @@
     unique-filename "^1.1.1"
     which "^1.3.1"
 
+"@github/clipboard-copy-element@^1.1.1":
+  version "1.1.1"
+  resolved "https://registry.yarnpkg.com/@github/clipboard-copy-element/-/clipboard-copy-element-1.1.1.tgz#5d58f970ab89a5ee86b402b04f7765e324b8089b"
+  integrity sha512-+kIpoWk2INRk8r22q/8GPgS6KG+YyWwmIfSbKuMgzKXt0cyt6m5K3HG2sZiVVWSVqZ458cLU74JrOJRevXvxQA==
+
 "@hapi/address@2.x.x":
   version "2.1.2"
   resolved "https://registry.yarnpkg.com/@hapi/address/-/address-2.1.2.tgz#1c794cd6dbf2354d1eb1ef10e0303f573e1c7222"