docs: remove references to play.etcd.io

[Milkjuiceman]

Closes etcd-io/etcd#1166
Removed all references to play.etcd.io as the playground is no longer being maintained:

• Removed the "Play" navigation link from config.yaml
• Removed the play.etcd.io URL ignore rule from .htmltest.yml

References in pre-v3.5 blog posts and old documentation were left untouched per the issue instructions.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @Milkjuiceman. 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.

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[jberkus]

This PR doesn't touch the majority of references to Play.

[jberkus]

Most of the references to play are missing here.