[kpal81xd]

Adds a user manual section for PlayCanvas Stage 0.1.0, listed under Getting Started as Use PlayCanvas Stage, beside Start with create-playcanvas and Use PlayCanvas Skills. URLs stay at /user-manual/stage/ so the section can move later without breaking links.

The home page follows the other product home pages: an intro, the promo video, Get Started and GitHub buttons, a five-minute quick start, "Where do you want to start?" cards and a diagram of how Stage fits together. Five sub-pages cover Installation (requirements, install, new scene, existing app), Editing Scenes, Comments, Devices and Inspector, and Configuration (Claude Code and Codex, settings, stopping the server, data and network use). Limitations and cautions are in admonition blocks on the pages they apply to.

It also adds a Stage llms.txt index, lists it under "Starting a project" in the root index, and links Stage from the Getting Started overview and Developing with AI. UI strings are quoted from the Stage source. The Japanese mirror is not updated, as with #1295.

It also fixes Mermaid edge labels in dark mode site-wide: Mermaid's dark theme drew each label on a grey box, so src/css/custom.scss now gives them the page background.

I confirm I have read the contributing guidelines.

Smoke test

1. Open /user-manual/stage/ → the page loads with the title "PlayCanvas Stage", and the sidebar shows a Use PlayCanvas Stage category under Getting Started, after Use PlayCanvas Skills, with five pages.
2. On the home page, play the video, then click Get Started → and each of the four cards → each opens the matching page or section, and the "How it fits together" diagram renders.
3. Open Installation → the Claude Code and Codex tabs switch, and the tip, note and warning blocks render.
4. Open Editing Scenes, Comments and Devices and Inspector → every clip plays (autoplay, muted, looping).
5. Open Configuration → the caution, info and warning blocks render.
6. Open /user-manual/engine/developing-with-ai/ → "Recommended Plugins and MCP Servers" lists PlayCanvas Stage and links to its home page.
7. Open /llms.txt → "Starting a project" lists Stage, and /user-manual/stage/llms.txt lists all six pages.