Fix `npm run docs-github` script. (#23688)

`$npm_config_tmp` is a non-existent variable.
Also, use double quotes since single ones don't work.
Finally, git ignore the temp file.

.gitignore

@@ -44,8 +44,9 @@ scss-lint-report.xml
 # grunt-contrib-sass cache
 .sass-cache
 
-# Jekyll metadata
+# Jekyll metadata and extra config file for `github` script
 docs/.jekyll-metadata
+twbsconfig.yml
 
 # Folders to ignore
 bower_components

package.json

@@ -43,7 +43,7 @@
     "docs-lint": "htmllint --rc build/.htmllintrc _gh_pages/*.html _gh_pages/**/*.html js/tests/visual/*.html",
     "docs-compile": "bundle exec jekyll build",
     "docs-serve": "bundle exec jekyll serve",
-    "docs-github": "shx echo 'github: true' > $npm_config_tmp/twbsconfig.yml && npm run docs-compile -- --config _config.yml,$npm_config_tmp/twbsconfig.yml && shx rm $npm_config_tmp/twbsconfig.yml",
+    "docs-github": "shx echo \"github: true\" > twbsconfig.yml && npm run docs-compile -- --config _config.yml,twbsconfig.yml && shx rm ./twbsconfig.yml",
     "docs-upload-preview": "build/upload-preview.sh",
     "maintenance-dependencies": "ncu -a -x jquery && npm update && bundle update && shx echo 'Manually update assets/js/vendor/*, js/tests/vendor/*, bower.json and .travis.yml'",
     "release-version": "node build/change-version.js",