Enhance the documentation and add markdown lint

- Add npm package.json to make easier maintaining dependencies.
- Make markdown files consistent using remark.
- Wrap markdown lines to 100 characters.
- Better README.md layout and visual.
- Enable VuePress landing/home page.
- Upgrade to latest VuePress version.
- Add link to the new Qt Marketplace.

docs/.vuepress/styles/index.styl

@@ -0,0 +1,37 @@
+@import 'palette.styl'
+
+a
+    svg.icon.outbound
+        margin-bottom -2px
+        margin-left 2px
+
+    img + svg.icon.outbound
+        display none
+
+sup
+    font-size 0.75em !important
+
+.home .hero
+    img
+        margin 1.5rem auto !important
+    .description
+        display none !important
+    #main-title
+        display none !important
+
+.content a code
+    color $accentColor
+
+.navbar
+    -webkit-box-shadow 0 4px 4px -4px rgba(0,0,0,0.1)
+    -moz-box-shadow 0 4px 4px -4px rgba(0,0,0,0.1)
+    box-shadow 0 4px 4px -4px rgba(0,0,0,0.1)
+
+    .logo
+        margin-right: 0.5rem;
+
+    .site-name
+        font-size 1.25rem
+
+    .repo-link
+        margin-left 1rem

docs/.vuepress/styles/palette.styl

@@ -0,0 +1,2 @@
+$accentColor = #23b223
+$textColor = #404244