Build and deploy storybook static files to GitHub pages for develop and master

Feature Description

We should make Storybook available on google.github.io/site-kit-wp.

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Acceptance criteria

• When pushing to develop or master, Storybook static files should be built and written to the gh-pages branch.
• In the gh-pages branch, a folder should be added per branch (e.g. so that the URLs will end up like https://google.github.io/site-kit-wp/storybook/develop and https://google.github.io/site-kit-wp/storybook/master).
• This must not happen for pull-requests.

Implementation Brief

There is an easy-to-use Travis-CI integration for deploying to GitHub pages (which e.g. the Web Stories plugin is using: https://github.com/google/web-stories-wp/blob/d181a4ea75cff53db730920fb41c41e2624a0c96/.travis.yml#L130-L141). However, that appears not to support deploying only partial content - you can specify a local_dir, but not a remote_dir. Hence, using a GitHub action workflow makes more sense. A lot of the necessary code can be taken from the existing build-pr.yml workflow.

• Add npm run build:storybook command that stores storybook in a storybook-static folder (ignore that folder in git).
• Add build-storybook.yml workflow that runs npm run build:storybook, then uploads the files as artifacts, then downloads it and integrates them into the gh-pages branch (in a storybook/{$branch_name} folder) and commits.
• Merge #1457.

QA Brief

Changelog entry

• N/A

[aaemnnosttv]

QA ❌

Currently no static assets are rendering in the build due to moving the files into a subdirectory; Storybook seems to assume that it’s running relative to the root rather than /storybook.

[felixarntz]

@aaemnnosttv I like your suggestion to use a subdirectory for storybook. I’ll update the IB (and PR). In terms of Netlify, I’d rather rely on GH Pages for now since that certainly works and is a thing we already have anyway, rather than adding another piece to our stack (may also be complicated because of google org requirements).