Provide a virtual staging area for assembled docs

[KOTungseth]

Description

Allow writers to preview assembled docs from a mix of current and WiP branches.

• Create previews that reflect final navigation, URLs, and redirects.
• Create easily shareable previews, such as temporary staging links.

Acceptance criteria

• Writers can assemble docs from a mix of current + WiP branches across multiple repos.
• The staging environment reflects final assembled output (URLs, navigation, redirects).
• Writers can validate changes across multiple repos without waiting for production deploys.