docs: Relates to Issue #995. Fix links in existing documentation (#999)

* docs: Remove safe-mix since is no longer local dependency

* docs: Fix existing packages link. Still only includes Substrate Core

substrate/core/sr-primitives/src/lib.rs

@@ -14,8 +14,10 @@
 // You should have received a copy of the GNU General Public License
 // along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
 
+// tag::description[]
 //! System manager: Handles all of the top-level stuff; executing block/transaction, setting code
 //! and depositing logs.
+// end::description[]
 
 #![cfg_attr(not(feature = "std"), no_std)]