docs(feed): add the cover image to the special missings post

Cover art for "28 Ways to Be Missing in SAS": a pack of wolves on a snow plain
at dusk with one animal standing apart, carrying the post's opening line - a
numeric missing is not a lone wolf, there are 28 of them.

Source image was 4:3, so it is cropped to 1.91:1 (1200x627) to match the other
feed covers and double as the LinkedIn share card. The crop was chosen to keep
both the lone wolf on the left and the full pack on the right in frame, losing
only sky above the clouds and foreground snow.

Sets previewImg in the front matter; the template renders it, so it is not
embedded in the body as well. Verified through gatsby build - the image
pipeline emits 300/600/1200-wide variants.

content/feed/sas-special-missings/index.md

@@ -7,6 +7,7 @@ authorLink: https://www.linkedin.com/in/allanbowe/
 tags:
   - Special Missings
   - Data Quality
+previewImg: './cover.jpeg'
 ---
 
 # 28 Ways to Be Missing in SAS