Redirect doc/ to docs

There is an alias in `content/docs/_index.html` for that redirection,
that is true. Such aliases only work if there is no corresponding
directory in `content/`, however, otherwise the `list` template kicks in
(which requires a special-form `_index.html` to redirect).

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

content/doc/_index.html

@@ -0,0 +1,3 @@
+---
+redirect_to: docs
+---