chore: add prepare-docs / build-docs, and run them in CI

[Naruto]

Adds the documentation build pair every repository in the family now shares, and points pages.yml at it.

scripts/prepare-docs.sh / .ps1	.venv + the pins in docs/requirements.txt
scripts/build-docs.sh / .ps1	English then Japanese, both --strict

Both take key=value options and --help, like the other scripts in scripts/:
prepare-docs.sh python=py force=yes, build-docs.sh locale=ja venv=.venv.

Why the -docs suffix

prepare and build are names the code build wants in several of these repositories, so the documentation pair carries -docs and is the same four names everywhere. SpriteStudio-Docs renamed its own pair to match.

What build-docs gets right

• Locale order. The English build clears site/, and the Japanese site lives inside it at site/ja — so English first, Japanese second. The opposite order publishes a site with no Japanese pages at all. locale=en on its own warns that it just deleted site/ja.
• --strict, always. Nothing builds the docs on a pull request, so a local run is the only gate there is.

CI

pages.yml calls the two scripts instead of repeating pip install -r + two zensical build lines. With no PR build there is nothing to catch CI and the README drifting apart, so the build is now spelled out in one place. build-docs.sh falls back to zensical on PATH when there is no venv, so the old shape would still have worked — running prepare-docs.sh keeps CI on exactly what a contributor runs.

Validation

prepare-docs.sh then build-docs.sh — both locales, --strict, No issues found. The .ps1 twins were run under pwsh, a bad option exits 2, and the locale=en warning fires. Nothing builds the docs on a PR, so that local run is the gate.