.github: install missing glightbox plugin for mkdocs

This is for automatic image zoom.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

.github/workflows/docs.yml

@@ -40,6 +40,7 @@ jobs:
           pipx inject mkdocs mkdocs-material
           pipx inject mkdocs pymdown-extensions
           pipx inject mkdocs mkdocs-callouts
+          pipx inject mkdocs mkdocs-glightbox
 
       - name: Build documentation
         run: mkdocs build --clean