feat: Site Previews for PR Reviews

[grayside]

This change introduces PR Previews based on research done in https://github.com/grayside/gh-page-previews.

Key changes to the github_pages workflow:

• No longer runs on Pull Requests, if this was meant as a build test, the preview deploy should be configured instead.
• Can now be run manually via the repo's Actions UI.
• Does not delete files from the gh-pages branch, if we add a file to the site and later delete it, this may require manual action. This may require follow-up to account for uniquely named static resources, such as CSS files.

Preview resources are entirely cleaned up when the PR is closed.

[github-actions[bot]]

🚀 Preview at https://GoogleCloudPlatform.github.io/samples-style-guide/previews/PR-46/

[grayside]

I did some additional digging. When docsy has changes to the underlying JS or SCSS, we will have additional files for minification. They are kept side by side to ease transition where HTML was cached but not the other static assets.

Since this doesn't change with content, I think it's reasonable to have periodic manual cleanup.

[github-actions[bot]]

🔎 Preview at https://GoogleCloudPlatform.github.io/samples-style-guide/previews/PR-46/

[github-actions[bot]]

🔎 Preview at https://GoogleCloudPlatform.github.io/samples-style-guide/previews/PR-46/

[github-actions[bot]]

🔎 Preview at https://GoogleCloudPlatform.github.io/samples-style-guide/previews/PR-46/

[github-actions[bot]]

🔎 Preview at https://GoogleCloudPlatform.github.io/samples-style-guide/previews/PR-46/

[github-actions[bot]]

🔎 Preview at https://GoogleCloudPlatform.github.io/samples-style-guide/previews/PR-46/

[github-actions[bot]]

🔎 Preview at https://GoogleCloudPlatform.github.io/samples-style-guide/previews/PR-46/

[github-actions[bot]]

🧨 Preview deployments removed.