vX.Y.Z doc pages with links into GitHub should link to vX.Y.Z not master

[chalin]

For example, consider the following link from etcd v3 API under v3.3.12:

• Avoid using links to master because these can break over time:
  https://github.com/coreos/etcd/tree/master/contrib/recipes, which no longer exists
• Instead link to https://github.com/etcd-io/etcd/tree/v3.3.12/contrib/recipes, which is valid

Related: #252

/tag cleanup