Documentation for older versions

[vidartf]

Do you want to request a feature or report a bug?
Documentation feature.

What is the current behavior?

The documentation of yarn always displays the docs for the latest version.

What is the expected behavior?

Sometimes you want to know e.g. when a specific command or flag was added, or behavior changed slightly. Sometimes you want to see the docs for a specific version of yarn. The documentation of Python allows you to use a select box to pick the Python version to see the docs for. There are also helpers in the docs like "Added in version 3.6". Either of these would help a lot. This would be especially useful since the CLI --help output is not always very helpful, but instead says "Visit https://yarnpkg.com/en/docs/cli/audit for documentation about this command.", which might be inaccurate (e.g. my older version of yarn does not seem to support the --level flag for yarn audit).