Remove old docs, rename docs_new to docs

It would be probably good idea to also remove tools/docgen.py and
asciidoc.mk, but asciidoc.mk is still mentioned under `cloud/doc`, which
may contain some useful info which we'll need at least to review before
removing.

PUBLISHED_FROM=faf454d4c52a2f07ea8ac084cf0bd11a0c9c9b3b

docs/usage-example/items.json

@@ -0,0 +1,5 @@
+{
+  "items": [
+    { "type": "markdown", "name": "intro.md" }
+  ]
+}