How to “Investigate rock arches on the Mun” in Kerbal Space Program

You can do this with Action Groups.

This youtube tutorial provides a good example of how to do exactly what you want:

In short, once you have the solar panels added to your ship:

1. Click on the "Action Groups" icon at the top of the screen. It's the blue button that looks like two gears.

2. Select "Custom1" or whichever number you would like the action to bind to.

3. Click on the solar panels you would like to extend/retract when the key is pressed.

4. Then select "Toggle Panels" (or "Extend Panels"/"Retract Panels", if you only want this action to work in one direction.)

Now when your ship is launched, you can use whichever number key you bound this action group to to toggle your solar panels.

According to their blog, the latest version of the demo is based off of 0.18.2. This means you have docking, which was added in .18. There are some restrictions from the full version, though:

• Part loading limited to the 25 stock ones.
• No Plugin loading.
• SPH and Runway Facilities are inaccessible.
• Only one save profile allowed.
• Cannot launch new flights if 3 or more are already in progress.
• Kerbin, the Mun and the Sun are the only celestial bodies in the solar system.

You should be able to see the version in the bottom right corner of the main screen, once the game has loaded. Currently, you're at least two versions behind.