Remove docs and replace by placeholders that point to the new locations

Author: dpvc

docs/.gitignore

@@ -0,0 +1,11 @@
+The source files for the documentation are now kept in a separate
+GitHub repository at
+
+       https://github.com/mathjax/mathjax-docs
+
+The HTML versions are now available at
+
+       http://mathjax.readthedocs.org/en/latest/
+
+where it is possible for you to submit corrections and modifications
+directly to the documentation on line.