[task/TOF] Move theme files into theme/, phases 0+1

[chalin]

Summary

• Contributes to Finalize monorepo setup - 26Q2 #2617
• Lands the first chunk of the Theme-Only Folder (TOF) Finalize monorepo setup - 26Q2 #2617 work
• Moves the Docsy theme files from the repo root into theme/.
• Updates docsy.dev to consume the new layout with a one-line Hugo config change.
• Adds planning and spike documentation under tasks/0.16/repo-reorg/.

Part of #2617. Design and sequencing: theme-only-folder.plan.md, theme-only-folder.execution.md.

PR-checks and preview status

• Ok for smoke and sanity checks to fail. They'll be addressed in the next phase, phase 2.
• Netlify deploy preview for builds successfully.
• Preview: https://deploy-preview-2639--docsydocs.netlify.app spot-check

Details

What this PR changes

• Creates theme/ and moves canonical theme assets (assets, i18n, images, layouts, static, hugo.yaml, theme.toml, go.mod, go.sum) into it.
• Updates theme/go.mod to module path github.com/google/docsy/theme.
• Simplifies theme/hugo.yaml mounts to canonical node_modules/<pkg> form (drops legacy ../../node_modules/* escapes).
• Adds theme/package.json as a private mirror of theme runtime deps, plus scripts/sync-theme-deps.mjs (and tests) to keep it aligned with the repo root.
• Updates repo-root maintainer scripts, format/lint globs, and make-site.sh paths for the new layout.
• Changes docsy.dev consumer config from theme: [docsy] to theme: [docsy/theme]; wires _install-theme-deps postinstall for cross-folder npm install (no root workspaces yet).
• Adds tasks/0.16/repo-reorg/ planning materials and spike-notes.md with consumer and maintainer change log.

Out of scope / follow-ups on the task branch

• Does not merge to main; targets the integration branch only.
• Leaves Phases 2–3 open: make-site.sh smoke runs (NPM, Hugo module, non-module), full GitHub CI matrix, and Netlify deploy-preview confirmation.
• Defers docsy-example validation and release-note drafting (Phases 4–5).