Create appmodel.md

[davidfowl]

Added the app model guide to the repo. This is great for LLMs to consume.

[Copilot]

Pull Request Overview

Adds a comprehensive application model guide to the Aspire documentation, covering concepts, design patterns, and authoring guidance.

• Introduces a Quick Start section with sample code and a Mermaid diagram.
• Provides in-depth explanations of resources, built-in types, interfaces, API patterns, and full examples (Redis, Talking Clock).
• Includes a glossary and reference tables for annotations, events, and value types.

Comments suppressed due to low confidence (4)

docs/specs/appmodel.md:648

• [nitpick] Use 'csharp' for code fences instead of 'C#' to match other examples and standard Markdown conventions.

```C#

docs/specs/appmodel.md:784

• [nitpick] This heading is level 2 inside a subsection; consider using '###' to maintain the nested section hierarchy under 'Referencing Endpoints from Other Resources'.

## EndpointReferenceExpression – accessing endpoint parts

docs/specs/appmodel.md:126

• [nitpick] Resource names differ from the Quick Start section (here using 'backend' vs. 'api'). Consider aligning names across examples for consistency.

var api = builder.AddProject("backend").WithReference(pg);

docs/specs/appmodel.md:1

• [nitpick] This document exceeds 1500 lines; consider splitting into smaller, focused files or sections to improve maintainability and navigation.

# Aspire Resource Model: Concepts, Design, and Authoring Guidance