New ebuild for pre-built documentation.

Package-Manager: portage-2.2.8-r2
sage-doc and sage-doc-bin should block each other

Package-Manager: portage-2.2.8-r2
sage 6.4 can use either sage-doc or sage-doc-bin

Package-Manager: portage-2.2.8-r2
make sage-doc-bin the default in 6.4