docs: add auto model selection and document file attachment (v1.0.32)

- Chapter 01: Add note about 'auto' model option so beginners don't
  have to manually pick a model — Copilot will choose the best one
- Chapter 02: Add 'Attaching Document Files' section explaining how
  to attach PDFs, Word docs, and other files to prompts for context

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

Author: DanWahlin

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

@@ -486,6 +486,8 @@ copilot
 
 > 💡 **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.
 
+> 💡 **Not sure which model to pick?** Select **`auto`** from the model picker to let Copilot automatically choose the best available model for each session. This is a great default if you're just getting started and don't want to think about model selection.
+
 </details>
 
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02-context-conversations/README.md

@@ -687,6 +687,21 @@ copilot
 
 > 📖 **Learn more**: See [Additional Context Features](../appendices/additional-context.md#working-with-images) for supported formats, practical use cases, and tips for combining images with code.
 
+### Attaching Document Files
+
+In addition to the `@` syntax and image paste, you can attach supported document files (such as PDFs, Word documents, and text files) directly to your prompts. This lets Copilot read and reason about external reference material — for example, a requirements spec, a design document, or a README from another project.
+
+When you start a prompt, look for the attachment option in the CLI interface to attach a file from your filesystem. Copilot will read the document's content and incorporate it into its response.
+
+```bash
+copilot
+
+# After attaching a document file (e.g. requirements.pdf), ask:
+> Based on the requirements I attached, what changes need to be made to @samples/book-app-project/book_app.py?
+```
+
+> 💡 **When is this useful?** Use document attachment when you have reference material that isn't already in your codebase — like a design spec, an API contract, or a brief from a stakeholder. It's faster than copy-pasting large documents into your prompt.
+
 </details>
 
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