Format the Markdown with prettier in make fmt and make fmt-check (closes #215)

make fmt and make fmt-check covered only Go, so Markdown formatting was
checked by eye. prettier, pinned in package.json and yarn.lock beside
ESLint and installed by the same js-deps stage, now formats every
Markdown file with the settings in .prettierrc (4-space tabs, prose
wrapped). It runs only in Docker: make fmt writes the formatted files
back from the markdown-output stage, and make fmt-check and the image
build run the markdown-check stage. The Dockerfile's lint stage now
runs the gofmt check itself, since make fmt-check needs a docker daemon.

Model: opus-5-5

.prettierrc

@@ -0,0 +1,4 @@
+{
+    "tabWidth": 4,
+    "proseWrap": "always"
+}