MINOR: [R] Add new vignette to pkgdown config (#49145)

### Rationale for this change

CI failing on preview-docs; see #49141

### What changes are included in this PR?

Add the vignette created in #49068 to pkgdown config

### Are these changes tested?

I'll trigger CI

### Are there any user-facing changes?

Nah

Authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>

Author: thisisnic

r/_pkgdown.yml

@@ -150,6 +150,7 @@ articles:
       - developers/docker
       - developers/install_details
       - developers/data_object_layout
+      - developers/binary_features
 
 reference:
 - title: Read datasets