Include GitHub Pages workflow in export

This workflow is intended to be useful to learners who have forked these
demos as a basis for their own modified games. But rather than using an
actual fork, they will likely have downloaded the project from the asset
library and committed it to a fresh repository.

https://phabricator.endlessm.com/T35699

Author: wjt

.gitattributes

@@ -1,9 +1,13 @@
 # Normalize EOL for all files that Git considers text files.
 * text=auto eol=lf
 
-# Exclude GitHub Actions-related files from published asset
-/.github                  export-ignore
-/asset-template.json.hb   export-ignore
+# Exclude GitHub Actions-related files from published asset, except the GitHub
+# Pages workflow which learners may find useful to publish their modified game.
+/.github/**                           export-ignore
+/.github/workflows                   !export-ignore
+/.github/workflows/**                 export-ignore
+/.github/workflows/githuh-pages.yml  !export-ignore
+/asset-template.json.hb               export-ignore
 
 # Exclude artwork sources
 /asset-sources            export-ignore