[docs] Adding manifest.json for handbook structure

[fellyph]

This commit introduces a new manifest.json file that outlines the structure and content of the WordPress Playground documentation. It includes sections for the handbook, guides, contributing, resources, and blueprints, each with associated titles, slugs, and markdown sources. This addition aims to enhance the organization and accessibility of the documentation.

[Copilot]

Pull request overview

This PR introduces a manifest.json file to define the organizational structure of WordPress Playground documentation, including handbooks, blueprints, and developer resources. The manifest establishes a hierarchical content structure with metadata for titles, slugs, markdown sources, parent-child relationships, and ordering.

Changes:

• Added a new manifest.json file that catalogs all documentation pages with their structural relationships and source locations

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[ashfame]

Findings:

• High: packages/docs/site/manifest.json omits documentation pages that are already present in sidebars.js, so consumers that use this manifest as the handbook structure would silently drop existing pages. Missing sidebar pages include main/troubleshooting.md, main/guides/php-code-snippets.md, main/guides/agent-skill-wp-playground.md, main/guides/programmatic-playground-cli.md, main/guides/phpunit-testing.md, main/guides/e2e-testing-with-playwright.md, developers/06-apis/sites-api/01-index.md, developers/06-apis/javascript-api/07-php-extensions.md, developers/06-apis/javascript-api/08-build-php-extensions.md, and developers/06-apis/javascript-api/09-php-extension-dependencies.md.

• Medium: the Blueprint navigation order in the manifest differs from sidebars.js: the manifest lists blueprints/using-blueprints before blueprints/data-format, while the current sidebar orders data-format before using-blueprints. That would produce a different handbook navigation order than the docs site.

Validation: the manifest parses as JSON, and npm exec nx run docs-site:check-orphan-pages passes against the current sidebar/docs set.

[ashfame]

@fellyph Feel free to use the above findings as a heads up and then proceed.

[akirk]

Lets merge it!