feat(html pipe): Default pipeline should include <section/>s (#379)

Include proper sections in the HTML pipeline instead of plain `<hr>` sectioning

BREAKING CHANGE:
- Remove `context.content.sections` and move the sections directly under
`context.content.mdast`
- Adjust json schemas accordingly
- Move the section yaml meta info to `<node>.meta`
- Move the section types to the mdast `<node>.meta.types`

Is dependent on adobe/helix-cli#1037

fix #279

2images.json

@@ -0,0 +1,41 @@
+{
+  "type": "root",
+  "children": [
+    {
+      "type": "paragraph",
+      "children": [
+        {
+          "type": "image",
+          "title": null,
+          "url": "./helix_logo.png",
+          "alt": "helix-logo"
+        },
+        {
+          "type": "text",
+          "value": "\n"
+        },
+        {
+          "type": "image",
+          "title": null,
+          "url": "./helix_logo.png",
+          "alt": "helix-logo"
+        }
+      ],
+      "meta": {
+        "types": [
+          "has-image"
+        ]
+      }
+    }
+  ],
+  "meta": {
+    "types": [
+      "has-image",
+      "nb-image-2",
+      "has-only-image"
+    ]
+  },
+  "title": "helix-logo\nhelix-logo",
+  "intro": "helix-logo\nhelix-logo",
+  "image": "./helix_logo.png"
+}