Refactor build to cake's new node based vitepress theme package

[josbeir]

Migrates the docs build from a custom bash dev server to a proper Node.js package setup using @cakephp/docs-skeleton.

• Add package.json with @cakephp/docs-skeleton and vitepress dependencies
• Replace custom bin/dev-server.sh with standard npm run docs:dev
• Move toc_*.json files into .vitepress, CI scripts into .github
• Update workflow, README, and contributor docs to reflect new paths

npm install && npm run docs:dev

Notes:

• When we merge this upwards to 5.next and 6.x keep an eye on refactored config file / toc locations which is now placed in the new .vitepress folder

[Copilot]

Pull request overview

This PR migrates the documentation tooling from a custom bash-based dev server to a Node.js/VitePress package setup built around @cakephp/docs-skeleton, aligning local development and CI with the upstream skeleton’s theme/config approach.

Changes:

• Adds package.json/package-lock.json and VitePress npm scripts for dev/build/preview.
• Moves TOC files into .vitepress/ and introduces a new .vitepress/config.js + theme entrypoint.
• Relocates CI helper scripts into .github/ and updates docs/CI references accordingly.

Reviewed changes

Copilot reviewed 9 out of 16 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
README.md	Updates local development instructions to use npm/VitePress scripts.
package.json	Introduces Node package setup and VitePress scripts/dependencies.
package-lock.json	Locks dependency tree for reproducible installs.
docs/en/contributing/documentation.md	Updates contributor commands to new .github/ script paths.
config.js	Updates TOC loading paths to .vitepress/.
bin/dev-server.sh	Removes the old bash orchestration dev server.
.vitepress/toc_en.json	Adds TOC under .vitepress/ for English sidebar.
.vitepress/toc_ja.json	Adds TOC under .vitepress/ for Japanese sidebar.
.vitepress/theme/index.js	Switches theme to @cakephp/docs-skeleton.
.vitepress/config.js	Adds VitePress config extending the skeleton base config.
.gitignore	Ignores newly introduced public asset paths.
.github/workflows/docs-validation.yml	Updates CI to call the relocated .github/* validation scripts.
.github/check-toc-links.js	Adds TOC link validation script under .github/.
.github/check-links.js	Adds internal markdown link checker under .github/.

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