Added an external-docs example to show the basic use case

[tmeasday]

Demo of an external-docs app w/ a refactored preview (not containing "render to screen") parts.

To try it:

1. Run yarn start in the external-docs example to see an example of an CRA app that uses doc blocks w/o a full storybook runtime or any kind of build.

2. Run window.preview.onUpdateArgs({ storyId: 'examples-button--export-0', updatedArgs: {label :'changed' }}) to see an example of a message reaching the preview to update the args (you can use window.preview.channel too).

Of note:

• The blocks.tsx file only depends on Preview (currently exported by @storybook/preview-web but we may move it to its own package) which doesn't attempt to render the "current story" and has no dependencies on docs (thus react).

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[shilman]

@tmeasday looking really great except for the version issues

[shilman]

@tmeasday

------------------------------
▶️  building: external-docs
------------------------------
info @storybook/react v6.5.0-alpha.51
info 
info => Cleaning outputDir: /tmp/storybook/built-storybooks/external-docs
ERR! Error: No configuration files have been found in your configDir (/tmp/storybook/examples/external-docs/.storybook).
ERR! Storybook needs either a "main" or "config" file.
ERR!     at validateConfigurationFiles (/tmp/storybook/lib/core-common/dist/cjs/utils/validate-configuration-files.js:50:11)
ERR!     at loadCustomPresets (/tmp/storybook/lib/core-common/dist/cjs/utils/load-custom-presets.js:19:62)
ERR!     at loadAllPresets (/tmp/storybook/lib/core-common/dist/cjs/presets.js:346:131)
ERR!     at buildStaticStandalone (/tmp/storybook/lib/core-server/dist/cjs/build-static.js:101:48)
ERR!     at async buildStatic (/tmp/storybook/lib/core-server/dist/cjs/build-static.js:201:5)
ERR!  Error: No configuration files have been found in your configDir (/tmp/storybook/examples/external-docs/.storybook).
ERR! Storybook needs either a "main" or "config" file.
ERR!     at validateConfigurationFiles (/tmp/storybook/lib/core-common/dist/cjs/utils/validate-configuration-files.js:50:11)
ERR!     at loadCustomPresets (/tmp/storybook/lib/core-common/dist/cjs/utils/load-custom-presets.js:19:62)
ERR!     at loadAllPresets (/tmp/storybook/lib/core-common/dist/cjs/presets.js:346:131)
ERR!     at buildStaticStandalone (/tmp/storybook/lib/core-server/dist/cjs/build-static.js:101:48)
ERR!     at async buildStatic (/tmp/storybook/lib/core-server/dist/cjs/build-static.js:201:5)