Version code samples with doc versions

[AndrewJakubowicz]

Related to #1123

Context

Currently the samples that are shown within all the docs are unversioned.

Ideally the docs/v2 samples are using Lit 2, and docs/v3 are using Lit 3.

Close criteria

The unversioned site should use whatever the unversioned docs are using. Lit 2.x currently, while the versioned v3 docs should use Lit 3 pre-release.