docs: add Theme Elementary guides and Docusaurus workflow

[Adi-ty]

Description

Create a clear documentation journey for Theme Elementary and publish it through the shared Docusaurus workflow.

Technical Details

• Reworked README.md, DEVELOPMENT.md, and CONTRIBUTING.md as concise entry points.
• Added Getting Started, Initialization, Local Development, Features, and Scaffolding guides.
• Replaced the old Quick Start and internal-testing documentation with structured internal maintainer guidance.
• Added cleanup validation, dependency-development procedures, temporary-procedure tracking, and validation-result recording.
• Added Graphify maintainer guidance.
• Updated asset, HMR, and Tailwind documentation links and behavior.
• Added the shared Docusaurus documentation workflow, pinned to a reviewed revision.
• Configured the supported theme-elementary-v2 branch and documentation sidebar.

Checklist

• Provide a primary Getting Started path.
• Document acquisition, initialization, local development, checks, and production builds.
• Catalogue built-in, example, and optional functionality.
• Document manual extension and companion-plugin CPT/taxonomy examples.
• Keep maintainer-only procedures under docs/internal/.
• Add documentation build automation.
• Remove stale Quick Start and internal-testing references.
• Verify documentation links and anchors.

Fixes/Covers Issue

Fixes #768

Covers #769, #770, #771, #772, #773, #774, #775, #776