docs: Specify the command for activating prompt library from the command palette (#27007)

Quickfix of the docs as I read through and get familiar with the
assistant interface.
`prompt-library: toggle` does not appear to be a live command in
`cmd-shift-p` - instead I see `assistant: deploy prompt library`. This
change to the docs reflects that. It also notes that this command can
only be activated from within the assistant panel (the command is not
accessible from a standard editor panel).

Release Notes:

- N/A