How-To guide: base Kubernetes YAML

[willtsai]

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Description

Taking over https://github.com/radius-project/docs/pull/748 from @jasonviviano to add a how-to guide on applying a base Kubernetes YAML using Radius.

Issue reference

Fixes: #703

[willtsai]

consider using this example instead to illustrate this feature: https://github.com/willdavsmith/radius/blob/willdavsmith/0.30-release-fixes/test/functional/shared/resources/testdata/manifest/basemanifest.yaml

[github-actions[bot]]

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