Merge pull request #185 from buildpacks/document-platform

Add Platform documentation

content/docs/concepts/components/platform.md

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+title="Platform"
+weight=3
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+
+## What is a Platform?
+
+A `platform` uses a [lifecycle][lifecycle], [buildpacks][buildpack] (packaged in a [builder][builder]), and application source code to produce an OCI image.
+
+<!--more-->
+
+## Examples
+Examples of a platform might include:
+
+* A local CLI tool that uses buildpacks to create OCI images. One such tool is the [Pack CLI][pack]
+* A plugin for a continuous integration service that uses buildpacks to create OCI images. One such plugin is the [buildpacks][buildpacks-tekton] plugin in [Tekton][tekton]
+* A cloud application platform that uses buildpacks to build source code before deployment. One such platform is [kpack][kpack]
+
+## API
+The platform [specification][spec] details what a platform does, and how it interacts with lifecycles and builders. The current API version for the platform is `0.4`.
+
+[builder]: /docs/concepts/components/builder/
+[buildpack]: /docs/concepts/components/buildpack/
+[lifecycle]: /docs/concepts/components/lifecycle/
+[spec]: https://github.com/buildpacks/spec/blob/main/platform.md
+[pack]: https://github.com/buildpacks/pack
+[buildpacks-tekton]: https://github.com/tektoncd/catalog/tree/master/task/buildpacks
+[tekton]: https://tekton.dev/
+[kpack]: https://github.com/pivotal/kpack
+[api-version]: https://github.com/buildpacks/spec/blob/main/platform.md#platform-api-version