Make completion feature easier to find

[lornajane]

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

We have the redocly completion command to let users add autocomplete to their commands, but it's not well-known so some users don't find it. Let's make it easier for users to discover this existing feature.

Describe the solution you'd like

Add a message after installation telling the user about the command if they want autocomplete.

Describe alternatives you've considered

We talked about automatically installing the autocomplete but that's more difficult and not every user will want that. The existing behaviour is fine, but the purpose of the issue is to make it more findable.