feat: describe the Joe's "plan" command (joe#115)

Description

Describe new command, plan, which performs EXPLAIN without execution.

Related issue

joe#115 (closed)

Checklist

• the MR description has been reviewed
• this MR contains text changes and they have been reviewed OR there are no texts changes
• this MR contains GUI/CLI changes and they have been reviewed OR there are no GUI/CLI changes
• this MR contains API changes and they have been reviewed OR there are no API changes