Redirect legacy mongo shell landing page

Author: Dave

config/redirects

@@ -1795,5 +1795,6 @@ raw: /manual/core/wildcard -> ${base}/manual/core/index-wildcard/
 [v5.0-*]: /${version}/reference/method/db.isMaster -> ${base}/${version}/reference/method/db.hello
 [v5.0-*]: /${version}/reference/method/db.eval -> ${base}/${version}/reference/method/js-database
 [v5.0-*]: /${version}/reference/method/db.getProfilingLevel -> ${base}/${version}/reference/method/db.getProfilingStatus
+[v5.0-*]: /${version}/mongo -> ${base}/${version}/reference/mongo