MNT: Add sitemap.txt generation for gh-pages

.github/deploy-gh-pages.sh

@@ -8,12 +8,23 @@ fi
 
 [ "$GH_PASSWORD" ] || exit 12
 
+sitemap() {
+    WEBSITE='https://kernc.github.io/backtesting.py'
+    find -name '*.html' |
+        sed "s,^\.,$WEBSITE," |
+        sed 's/index.html$//' |
+        grep -v '/google.*\.html$' |
+        sort -u  > 'sitemap.txt'
+    echo "Sitemap: $WEBSITE/sitemap.txt" > 'robots.txt'
+}
+
 head=$(git rev-parse HEAD)
 
 git clone -b gh-pages "https://kernc:$GH_PASSWORD@github.com/$TRAVIS_REPO_SLUG.git" gh-pages
 mkdir -p gh-pages/doc
 cp -R doc/build/* gh-pages/doc/
 cd gh-pages
+sitemap
 git add *
 git diff --staged --quiet && echo "$0: No changes to commit." && exit 0
 git commit -a -m "CI: Update docs for $TRAVIS_TAG ($head)"