Preserve preview sites on master release

[michaeltaranto]

Whitelisting the preview (and grad-connection) folders to prevent the release of the docs site from cleaning the output directory and blowing them away.

Also opting into single-commit as an optimisation as we have no need to git history on the gh-pages branch.

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Latest commit: b09e482

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[michaeltaranto]

Planning to host the grad-connection branch on a custom folder path (like what we had on surge.sh previously).

https://seek-oss.github.io/braid-design-system/grad-connection/

This will provide a persistent URL for the team.