docs: add Project Structure guide page

[antfubot]

What

Adds a new guide page — Project Structure — documenting the recommended folder layout for a devframe package, distilled from reading through all the built-in plugins, the examples, and the starter/ template.

The page maps the shared convention: one defineDevframe factory drives every surface, so the layout's job is to keep the node and browser sides in separate module graphs and expose each seam (src/index.ts factory, src/node/ setup, src/rpc/ registry, src/client/ connect helper, src/spa/ UI, sibling --assets package, cli.ts + bin.mjs) through its own package subpath. It covers the tsdown dual-graph build, mount-path-portable SPA (base: './'), and the optional pieces (inject/, shared/, diagnostics.ts, playgrounds, tests).

Why

There was no single page describing how to lay out a devframe package end to end; authors had to reverse-engineer it from the plugins. This gives a canonical reference right after the Devframe Definition page.

Notes

• Slotted in as guide page 2 (right after "Devframe Definition"); the existing guide pages were renumbered accordingly (git tracked these as renames). Cross-links use path names, not numbers, so none needed updating.
• Linked from the intro's "What's next".
• Follows the repo doc style (positive framing, minimal callouts, [filename] code fences).

This PR was created with the help of an agent.

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
devframe	Ignored		Aug 25, 2026 2:33am