Add sample form for selecting a subset of items to rank

Follow on to https://github.com/getodk/docs/issues/1074 since it's a useful pattern that may not be immediately obvious.

Recommend making the select question required with at least two that need to be selected and only making the rank question relevant once some options have been selected.

Read the linked issue #1074 first to understand the existing pattern and where this documentation belongs. Add a sample form showing a required select question with at least two selections, with the rank question becoming relevant only after options are selected; done means the example clearly demonstrates this workflow.

Written by the indexing model from the issue text.