New content (guides/refs/updates to existing for LLM content gaps)

[hollyschinsky]

Description

Added

• New Add-on Architecture guide with dual-runtime system explanation, SDK imports and usage, and cross-runtime communication patterns.
• New Add-on Project Anatomy & CLI Templates guide with add-on project structure, file organization, and template selection for efficient Adobe Express add-on development.
• New Add-on Development Terminology guide with standardized definitions, decision matrices, and troubleshooting for Adobe Express Add-on development terminology.
• New practical usage guide for Using Add-on UI SDK Constants, import patterns (via addOnUISdk.constants.*) or named import), use case examples, etc
• New practical usage guide for Using Document Sandbox Constants to guide on the use of constants in the Document Sandbox environment, with use cases and practical examples.
• New "SDK Fundamentals" section in navigation under "Learn" to group terminology and constants usage guides for better discoverability.
• All new guides have FAQs and other helpful metadata updates for LLM/SEO.

Updated

• Restructure of the Add-on UI SDK Constants Reference - removed the old html table in favor of linkable sections for each
• Improved content and metadata for SEO and AI assistant optimization.

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.