add missing symlink (#1480)

Author: jeff-allen-mongo

config/redirects

@@ -6,6 +6,7 @@ symlink: stable -> v6.0
 symlink: rapid -> v6.0
 symlink: current -> v6.0
 symlink: upcoming -> v6.0
+symlink: manual -> v6.0
 
 [v2.2]: ${prefix}/${version}/core/read-operations-introduction -> ${base}/${version}/core/read-operations/
 [v2.2]: ${prefix}/${version}/core/write-operations-introduction -> ${base}/${version}/core/write-operations/