chore: remove references to Plugin SDK and update licensing

[kristinaquinones]

This pull request removes the Plugin SDK (GPLv3) and plugin system from the EmberDocs project, consolidating the licensing model to a proprietary source-available license for the core framework. All references to the Plugin SDK, plugin system, and related extensibility features have been removed from documentation, changelogs, architecture decisions, and project plans. The project now focuses solely on the proprietary core, with no separate open-source SDK or plugin extensibility in the current release phases.

Licensing Model Changes:

• Removed all references to the Plugin SDK (GPLv3) and its separate license from documentation, including README.md, LICENSE, DEVELOPMENT-STANDARDS.md, docs/EMBERDOCS-LICENSING.md, and brand guidelines. The licensing model is now solely a proprietary source-available license for the core framework. [1] [2] [3] [4] [5] [6] [7]

• Updated architecture decision records to remove the Plugin SDK boundary, plugin system hooks, and the dual-license context. The ADL-013 decision now only describes the proprietary core license, and the previously accepted ADL-006 and ADL-008 plugin-related decisions have been removed. [1] [2] [3] [4]

Feature & Roadmap Adjustments:

• Removed the plugin system and extensibility features from the project roadmap, changelog, feature dependencies, and summary documents. The MVP and Phase 02 are now focused on stabilization, accessibility, versioning, and documentation, with no plugin milestones. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

These changes clarify the project's licensing and scope, ensuring all documentation and planning accurately reflect the removal of the Plugin SDK and plugin system.- Removed all mentions of the Plugin SDK and its GPLv3 licensing from various documentation files.

• Updated licensing information to reflect the proprietary source-available model for the core framework.
• Revised documentation to enhance clarity regarding the licensing structure and compliance.
• Adjusted project planning documents to remove references to the plugin system, focusing on stabilization and version detection instead.