Add some foundations chapters

Author: southgate

slides/github-intermediate-advanced.md

@@ -9,6 +9,20 @@ chapters: [
   'trainers/matt-graham',
   'trainers/alex-southgate',
   'intro',
+  'github/intro',
+  'github/forking',
+  'github/pull-request',
+  'diff',
+  'log',
+  'rm-mv',
+  'ignore',
+  'branch',
+  'checkout',
+  'merge',
+  'rebase',
+  'reset',
+  'gui',
+  'reflog',
   'advanced/bisect',
   'advanced/bundle',
   'advanced/cherry-pick',