Add documentation to track all feature gates added.

[siyuanfoundation]

As part of etcd-io/etcd#18023, we need to add a feature gate page similar to the Kubernetes Feature Gates page to track all the feature gates added.

Current feature gates can be found in https://github.com/etcd-io/etcd/blob/main/server/features/etcd_features.go

[gangli113]

/assign

[ivanvc]

This was addressed by #941, closing. @siyuanfoundation, if you think otherwise please re-open this issue :)