Add documentation of versions bucket on main README and scoop install --help

[tghw]

Add documentation about using versions bucket on main README and scoop install --help

Is your feature request related to a problem? Please describe.

The workflow for installing older versions using the versions bucket is not clearly documented in the main install documentation, therefore users may not be aware of it. Currently, the only documentation of it exists in a subpage with specific packages and workflow documented.

Describe the solution you'd like

Add documentation for installing older versions of apps using the versions bucket to the root README and to scoop install --help. This should augment the documentation about the auto-generated manifests that is currently there, explaining the differences between the two approaches.

This issue was surfaced during the discussion of #5582 and is being opened as a separate ticket to avoid confusing the main issue on that ticket.