Launch documentation for Lit 3.0 pre-release

[AndrewJakubowicz]

Tracking issue for Lit.dev Lit 3.0 documentation.

See Lit 3.0 RFC https://github.com/lit/rfcs/blob/23422762cd7a6b41bed3a78644b2444e4eaa676b/rfcs/NNNN-lit-3.0.md

Umbrella issue for launching Lit 3.0 lit.dev documentation.

Requires:

• Upgrade our infra.
• Add unversioned generated pages from latest version docs: Lit.dev versioned documentation & automatic generation of latest unversioned docs. #1108
• Add v3 prerelease docs: Add Lit v3 pre-release documentation under docs/v3 #1109
  ~ - [ ] Launch v3 docs - flip latest version to v3. ~

[AndrewJakubowicz]

Done - we can flip docs to v3 as default as a separate issue when we are ready to release Lit 3.