versioned docc archive content

[heckj]

Set up and describe constraints to support presenting versioned content docs.swift.org

• DocC doesn't support (today) versioning internal to an DocC archive, so we need to wrap some structure around this, including choosing between versions being presented.
• Not all content should be versioned together, but some should be - and coordinated with versioned content from other repositories
• detail and document how this works - branching constraints, etc - and what the resulting URI structure needs to be
• explore options for redirecting to enable a "latest" auto-selection of documentation content for any versioned collection

[heckj]

with #72 in place, the merging of multiple archives is happening pretty smoothly, so earlier bugs that I saw aren't an issue. The JSON data in that structure identifies what should get merged together - leaving "swiftly" and the "vscode-swift" separate.