Add feature gates doc for etcd

[gangli113]

#940

This change copied most content from https://github.com/kubernetes/website/edit/main/content/en/docs/reference/command-line-tools-reference/feature-gates/index.md.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @gangli113. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[gangli113]

/cc @siyuanfoundation

[siyuanfoundation]

/lgtm Thanks @gangli113 !

/cc @serathius @ahrtr

[jmhbnz]

/ok-to-test

[spzala]

Please see inline review comment. Thanks!

[gangli113]

/retest

[spzala]

@gangli113 thanks for quickly addressing my comments, and sorry not being picky but the etcd project convention is to use smaller case etcd even when start a sentence. So I would suggest to change Etcd to etcd.