Deselect before selecting top faces + mouse options

After triangulation, all faces will be selected. So selecting the top two faces will just unselect them rather than the desired result of selecting them. Also added mouse options for some of the commands.
Encourage use of hot keys

Encourage use of hot keys (even though mouse commands are given).
Encourage use of hot keys

Needed a space before .._gen_mesh?
Begin putting description in front of images.

I'm going to experiment with putting the instructions ahead of the images so people can read what's about to be done before seeing it.
Begin putting description in front of images.
Continue putting descriptions in front of images.
Added quotes around literal references.
Continue putting descriptions in front of images.
Continue putting descriptions in front of images.
Continue putting descriptions in front of images.
Tried to be more explicit about panel names and locations.
Cleaned up the video / written instructions section.
Promoted titles and continued putting descriptions in front of images.
Continue putting descriptions in front of images.
Update getting_started.rst
Explicit use of "MDL files" rather than "MDLs".
Added advisory about needing to scroll panel.
Minor clarifications targeted at new MCell users.
Minor clarifications targeted at new MCell users.
Minor clarifications targeted at new MCell users.