Fix remaining broken external links

Follow up issue to https://github.com/etcd-io/website/issues/203, there are a handful of broken links which need attention -- there isn't an obvious place to link to, so the copy may need to be reworked.

docs/v3.4/learning/why/index.html
* Non-OK status: 404 --- docs/v3.4/learning/why/index.html --> https://godoc.org/github.com/coreos/etcd-io/etcdserver/api/v3election/v3electionpb

docs/v3.3/learning/why/index.html
* Non-OK status: 404 --- docs/v3.3/learning/why/index.html --> https://godoc.org/github.com/coreos/etcd-io/etcdserver/api/v3election/v3electionpb

docs/v3.1/libraries-and-tools/index.html
* Non-OK status: 404 --- docs/v3.1/libraries-and-tools/index.html --> https://github.com/scrz/scrz

docs/v3.1/op-guide/monitoring/index.html
* request exceeded our ExternalTimeout --- docs/v3.1/op-guide/monitoring/index.html --> http://dash.etcd.io/dashboard/db/test-etcd

docs/next/learning/why/index.html
* Non-OK status: 404 --- docs/next/learning/why/index.html --> https://godoc.org/github.com/etcd-io/etcd/server/etcdserver/api/v3lock/v3lockpb
* Non-OK status: 404 --- docs/next/learning/why/index.html --> https://godoc.org/github.com/etcd-io/etcd/server/etcdserver/api/v3election/v3electionpb
* Non-OK status: 404 --- docs/next/learning/why/index.html --> https://godoc.org/github.com/etcd-io/etcd/server/etcdserver/api/v3lock/v3lockpb

_Originally posted by @nate-double-u in https://github.com/etcd-io/website/issues/203#issuecomment-823642654_