Format Markdown with prettier in make fmt (closes #43)

script/fmt and script/fmt-check now run prettier on every Markdown file
after their Go steps, as the standard scripts from sneak/prompts at
dd4027b do. script/bootstrap gains the standard node, nvm and yarn
install next to its goimports install, and the last Dockerfile stage
copies package.json and yarn.lock before running it. .prettierrc,
.prettierignore and yarn.lock are the standard copies. package.json is
the standard one without its licence field and marked private, because
this repo's licence is the WTFPL. .dockerignore keeps the log of a failed
yarn install out of the build context. The existing Markdown is
reformatted in the next commit.

Model: opus-5-5

README.md

@@ -143,8 +143,10 @@ alpine. We provide:
 
 - `script/bootstrap` — install all dependencies (go if missing, goimports with
   `go install` at a pinned commit unless the installed one already reports the
-  pinned version, then `go mod download`); golangci-lint is not installed,
-  since it runs only in Docker
+  pinned version, then `go mod download`; node through nvm at a pinned version
+  if node is missing, yarn at a pinned version if it is missing, then the
+  `prettier` that `package.json` and `yarn.lock` pin); golangci-lint is not
+  installed, since it runs only in Docker
 - `script/setup` — set up the repo for development after a fresh clone: runs
   `script/bootstrap`, then `script/install-precommit`
 - `script/projectname` — output the project name (our own extension); used by
@@ -155,10 +157,10 @@ alpine. We provide:
 - `script/lint` — run golangci-lint in Docker by building only the `lint`
   stage of the `Dockerfile`, without the build cache, so every run lints; the
   image is tagged `simplelog-lint`
-- `script/fmt` — format all files (writes) with the goimports that
-  `script/bootstrap` installs
+- `script/fmt` — format all files (writes): Go with the goimports that
+  `script/bootstrap` installs, then every `*.md` file with `prettier`
 - `script/fmt-check` — check formatting (read-only); fails if `gofmt -l`
-  reports files
+  reports files, or if `prettier` would change any `*.md` file
 - `script/check` — run all checks: `test`, `lint`, `fmt-check` (our own
   extension)
 - `script/docker` — build the Docker image without the build cache, tagged