Version the Lighthouse docs

[kaycebasques]

Context

• core(audits): point perf audit description links to web.dev lighthouse#9540 (review)
• content: Update metrics tables and explanations #1631 (comment)

Problem

All of the Lighthouse versions point to the same docs on web.dev. If we include screenshots of the Lighthouse report UI, or if we mention that Lighthouse measures a certain number of metrics, then the doc will appear outdated to some Lighthouse users. The metrics might be in a different order, or the metrics might be different altogether. The implementations of the metrics change over time, too. This problem might apply to the rest of the audit documentation, but the problem is most visible in the metrics documentation.

Example of current UI:

Options

• Continue using one document per audit or metric, but add historical information to each document. Examples:
  • Mention what version of Lighthouse the screenshot was taken in and mention that if users are on a different version the UI will look different.
  • Add a Changelog section explaining how a metric or audit has changed over time.
• Create a versioning infrastructure to serve a different document depending on what Lighthouse version the user is running. A lot of the content will be the same so we'll need a good DRY infrastructure.

[kaycebasques]

This was more of a speculative idea. I don't think this work is actually going to happen.