feat(v2): docs version configuration: lastVersion, version.{path,label}

[slorber]

Motivation

Implement some low hanging fruits of #3285

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Unit tests

Dogfooding in dev env and deploy previews with a special config, as it makes more sense to focus on "next version" in those contexts:

          lastVersion: isDev || isDeployPreview ? 'current' : undefined,
          versions: {
            current: {
              path: isDeployPreview ? 'next-pr' : undefined,
              label: isDeployPreview ? 'Next (PR)' : 'Next',
            },
          },

[docusaurus-bot]

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