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

script/cibuild

@@ -1,8 +1,8 @@
 #!/bin/sh
-# script/cibuild: run the CI build. The Dockerfile runs the checks
-# (make fmt-check, lint, test), so a successful build implies a green
-# repo. Generic: needs no adaptation. The Gitea workflow runs this on
-# push.
+# script/cibuild: run the CI build. The Dockerfile runs the checks (the
+# gofmt check, golangci-lint, the stylesheet check, ESLint, the Markdown
+# check, make test), so a successful build implies a green repo. Generic:
+# needs no adaptation. The Gitea workflow runs this on push.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"