[NaCl SDK Docs] Move generated docs to native_client_sdk/doc_generated.

This makes it easier to grep native_client_sdk without grepping
generated data.

I also cleaned up generate_docs.py a little so it doesn't rely on
relative paths.

No trybots because this is a documentation-only change.

BUG=none
R=awatson@chromium.org, sbc@chromium.org

Review URL: https://codereview.chromium.org/145133002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246393 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246393