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-api/README.adoc

@@ -0,0 +1,12 @@
+= Runtime API
+
+.Summary
+[source, toml]
+----
+include::Cargo.toml[lines=2..5]
+----
+
+.Description
+----
+include::src/lib.rs[tag=description]
+----

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

@@ -14,7 +14,9 @@
 // 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[]
 //! API's for interfacing with the runtime via native/wasm.
+// end::description[]
 
 #![cfg_attr(not(feature = "std"), no_std)]