Add comprehensive authentication guide for API keys (#4701)

Co-authored-by: Claude <noreply@anthropic.com>

Author: drernie

docs/Quickstart.md

@@ -28,11 +28,29 @@ Explore production datasets with guided examples:
 pip install quilt3
 ```
 
-### 2. **Browse Public Data**
+### 2. **Authenticate (Optional for Public Data)**
+
+For public datasets like `s3://quilt-example`, no authentication is needed. For private buckets or catalogs, choose your authentication method:
+
+<!-- pytest-codeblocks:skip -->
+```python
+import quilt3
+
+# Interactive login (for local development, notebooks)
+quilt3.login()  # Opens browser for OAuth/SSO
+
+# OR use an API key (for automation, CI/CD, scripts)
+import os
+quilt3.login_with_api_key(os.environ["QUILT_API_KEY"])
+```
+
+**📚 Learn more**: See the [Authentication Guide](api-reference/authentication.md) for detailed setup instructions, best practices, and use cases.
+
+### 3. **Browse Public Data**
 ```python
 import quilt3
 
-# Browse available datasets
+# Browse available datasets (no auth needed for public data)
 packages = list(quilt3.list_packages("s3://quilt-example"))
 print(f"Found {len(packages)} public datasets")
 
@@ -41,7 +59,7 @@ pkg = quilt3.Package.browse("examples/hurdat", "s3://quilt-example")
 print(pkg)
 ```
 
-### 3. **Access Your First File**
+### 4. **Access Your First File**
 <!-- pytest-codeblocks:cont -->
 ```python
 # Download and read a file (using pkg from previous step)
@@ -50,7 +68,7 @@ content = data_file.get()
 print(content)
 ```
 
-### 4. **Create Your First Package**
+### 5. **Create Your First Package**
 ```python
 import quilt3
 import tempfile

docs/SUMMARY.md

@@ -62,6 +62,7 @@
   * [Local Catalog](Catalog/LocalMode.md)
   * [CLI, Environment](api-reference/cli.md)
   * [Known Limitations](api-reference/limitations.md)
+  * [Authentication Guide](api-reference/authentication.md)
   * [Custom SSL Certificates](api-reference/custom-ssl-certificates.md)
 * **Advanced**
   * [Browsing Buckets](walkthrough/working-with-a-bucket.md)