docs: generate page for metrics per version to replace plain .txt file

[christophsj]

Create pages for displaying a list of metrics per version, instead of linking to a plain .txt file.

This will make it much easier to read and navigate through the metrics.
The page is based on .txt file content, e.g.:

# name: "etcd_network_peer_round_trip_time_seconds"
# description: "Round-Trip-Time histogram between peers"
# type: "histogram"
etcd_network_peer_round_trip_time_seconds_bucket{To="REMOTE_PEER_NODE_ID",le="+Inf"}

will be parsed into a single row in the table with name, description, type.
Also, Labels within the metric (To="REMOTE_PEER_NODE_ID",le="+Inf" in this example) and one example value will be displayed.

metrics.md was renamed/moved to metrics/_index.md to make breadcrumb navigation work.

{{< metrics-list >}} is no longer necessary because the links to child pages will be automatically generated below the content.

Use simple_list: true to remove the separator between the heading Generated list of metrics and links to the versions

Issue #1133

[k8s-ci-robot]

Hi @christophsj. Thanks for your PR.

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[k8s-ci-robot]

@ronaldngounou: The /test command needs one or more targets.
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[ronaldngounou]

/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: christophsj, jberkus

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[jberkus]

Thanks! And let us know if you want to work on the long-term replacement.

[christophsj]

@jberkus definitely, would be glad to keep working on this