Update screenshots to those from modern Spyder, and add additional shots of different Main Window layouts, etc

[CAM-Gerlach]

Issue Description

Many of the screenshots we currently have are from very old versions of Spyder, running on vintage OS versions and have a lot of chrome around the actual content. This is both potentially inaccurate and confusing to our current users, and does not present a good impression to potential new ones. Therefore, this should be speedily corrected before the initial public release.

As part of spyder-ide/spyder-website#75 I will be taking some screenshots anyway, so I may as well hit on what is needed here. Specific work items:

• Add "default" master screenshot to Index and Overview pages
• Add ~3 additional screenshots with various panel, layout, and color arrangements to the Overview page
• Add/update panel screenshots for each respective page

Where is the relevant portion of the documentation located?

Primarily in the sections on the core/older features, such as the editor and console, and in sections not referring to a specific feature.

What would you suggest it be replaced with?

More and better screenshots from modern Spyder.