Convert site to Hugo (#8316)

This commit converts content and layout to use Hugo.

content/en/docs/templates/feature-state-alpha.txt

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+This feature is currently in a *alpha* state, meaning:
+
+* The version names contain alpha (e.g. v1alpha1).
+* Might be buggy. Enabling the feature may expose bugs. Disabled by default.
+* Support for feature may be dropped at any time without notice.
+* The API may change in incompatible ways in a later software release without notice.
+* Recommended for use only in short-lived testing clusters, due to increased risk of bugs and lack of long-term support.
+

content/en/docs/templates/feature-state-beta.txt

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+This feature is currently in a *beta* state, meaning:
+
+* The version names contain beta (e.g. v2beta3).
+* Code is well tested. Enabling the feature is considered safe. Enabled by default.
+* Support for the overall feature will not be dropped, though details may change.
+* The schema and/or semantics of objects may change in incompatible ways in a subsequent beta or stable release. When this happens, we will provide instructions for migrating to the next version. This may require deleting, editing, and re-creating API objects. The editing process may require some thought. This may require downtime for applications that rely on the feature.
+* Recommended for only non-business-critical uses because of potential for incompatible changes in subsequent releases. If you have multiple clusters that can be upgraded independently, you may be able to relax this restriction.
+* **Please do try our beta features and give feedback on them! After they exit beta, it may not be practical for us to make more changes.**

content/en/docs/templates/feature-state-deprecated.txt

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+
+This feature is *deprecated*. For more information on this state, see the [Kubernetes Deprecation Policy](/docs/reference/deprecation-policy/).

content/en/docs/templates/feature-state-stable.txt

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+
+This feature is *stable*, meaning:
+
+* The version name is vX where X is an integer.
+* Stable versions of features will appear in released software for many subsequent versions.

content/en/docs/templates/index.md

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+---
+headless: true
+
+resources:
+- src: "*alpha*"
+  title: "alpha"
+- src: "*beta*"
+  title: "beta"
+- src: "*deprecated*"
+  title: "deprecated"
+- src: "*stable*"
+  title: "stable"
+---