Document switching between dashboard runs on the Explore page

[JamesNK]

Description

Update the dashboard Explore documentation to show how to use the run selector introduced by dashboard data persistence.

The page should:

• Show where the run selector appears and how to switch between the current and completed runs.
• Explain that historical runs are read-only.
• Note that run selection is available when using Run persistence mode.
• Link to the dashboard data persistence documentation for details about modes and retention.

This is follow-up work for #1649.