Lit.dev versioned documentation & automatic generation of latest unversioned docs.

[AndrewJakubowicz]

Part of #1074

Motivated by:

• the need to add v3 prerelease documentation while keeping unversioned content stable and referencing v2.
• Intention that bookmarking an unversioned URL intends to bookmark the latest documentation - while bookmarking a versioned URL pins the version of the docs.

Design was initiated by #1083 (comment)

Proposal

A new Lit.dev documentation scheme where:

• All content is versioned in a subdirectory, e.g., v1, v2, and v, where N is all future major versions.
• Infrastructure where the current stable latest version is copied into the root docs folder to preserve unversioned (latest) URLs.

Design

• Move all currently unversioned (v2) documentation to a versioned subdirectory, and fix all cross-links so they are also versioned.
• Add build tooling that:
  • Copies the latest versioned docs content such that it is unversioned.
  • Add rel=canonical from the latest versioned pages to the unversioned page.
  • Fix all the cross links within the generated unversioned content such that all links are to other unversioned pages.

Result

After concluding this design all the current lit.dev URLs remain exactly the same. Except all unversioned pages, e.g., /docs/ will now have a versioned URL: /docs/v2/.

All lit.dev docs authoring will now happen in explicitly versioned directories.