docs: add auto model selection and document file attachment features

Document two new beginner-friendly features introduced in Copilot CLI v1.0.32:

- Chapter 01: Update Switching Models section to mention the 'auto' model
  option, which lets Copilot automatically pick the best available model
- Chapter 02: Add new 'Attaching Document Files' optional section explaining
  how to attach PDFs and other documents to prompts using the @ syntax

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: DanWahlin

01-setup-and-first-steps/README.md

@@ -481,9 +481,13 @@ Copilot CLI supports multiple AI models from OpenAI, Anthropic, Google, and othe
 copilot
 > /model
 
-# Shows available models and lets you pick one. Select Sonnet 4.5.
+# Shows available models and lets you pick one.
+# Select "auto" to let Copilot automatically choose the best model for each task.
+# Or select a specific model like Sonnet 4.5.
 ```
 
+> 💡 **Not sure which model to choose?** Select `auto` when prompted. Copilot will automatically pick the best available model for each session based on the task. It's the simplest option and a great starting point.
+
 > 💡 **Tip**: Some models cost more "premium requests" than others. Models marked **1x** (like Claude Sonnet 4.5) are a great default. They're capable and efficient. Higher-multiplier models use your premium request quota faster, so save those for when you really need them.
 
 </details>

02-context-conversations/README.md

@@ -427,6 +427,7 @@ These topics build on the essentials above. **Pick what interests you, or skip a
 | Token limits and `/compact` | [Understanding Context Windows](#understanding-context-windows) |
 | How to pick the right files to reference | [Choosing What to Reference](#choosing-what-to-reference) |
 | Analyzing screenshots and mockups | [Working with Images](#working-with-images) |
+| Attaching PDF and other document files | [Attaching Document Files](#attaching-documents) |
 
 <details>
 <summary><strong>Additional @ Patterns & Session Commands</strong></summary>
@@ -689,6 +690,28 @@ copilot
 
 </details>
 
+<details>
+<summary><strong>Attaching Document Files</strong></summary>
+<a id="attaching-documents"></a>
+
+### Attaching Document Files
+
+In addition to code files and images, you can attach supported **document files** (such as PDFs) to your prompts. This lets Copilot read and reason about the contents of documents alongside your code.
+
+```bash
+copilot
+
+> @requirements.pdf @samples/book-app-project/ Does the book app satisfy these requirements?
+
+> @architecture-diagram.pdf Summarize the key design decisions and check if @samples/book-app-project/ follows them
+```
+
+> 💡 **When is this useful?** Suppose you have a PDF of technical requirements, a design document, or a user manual. You can attach it directly so Copilot can understand the context without you having to copy-paste the contents manually.
+
+> ⚠️ **Note**: Not all document formats are supported. PDFs are a common format that works well. If Copilot cannot read a file, try converting it to a plain text or Markdown format first.
+
+</details>
+
 ---
 
 # Practice