Publish a preview / staging version of the book

[krilnon]

Per discussion in the docs workgroup, it'd be good to have a preview version of the DocC output that we can all look at as we work toward publishing the book on swift.org. This preview should be:

• Published on GitHub pages, probably using a workflow like this one from swift-docc-plugin.
• Visually distinguishable as a draft/preview version so we don't confuse people who are looking for the docs and not participating in the DocC conversion effort.

[krilnon]

To publish this either in a preview environment or on swift.org, we need custom header support. That exists with --experimental-enable-custom-templates, but it doesn't work when you also build with --transform-for-static-hosting, which we use for TSPL.

Tracked by https://github.com/apple/swift-docc/issues/196, and I have an initial fix drafted in https://github.com/apple/swift-docc/pull/410.