tutorial: add concluding section at end of guided tour

[jhpalmieri]

In response to (for example) the closing of ticket #6765, I think we should add a section at the end of the chapter "A Guided Tour" of Sage's tutorial, containing

• very short descriptions of some other features and capabilities of Sage, perhaps with pointers to more information. This should not be a laundry list of everything available, just some high points to catch readers' attention.

• similarly, short descriptions of some of the optional packages available for Sage (like the linear programming stuff). Again, we don't want to describe every optional package, just some high points.

• a few topics could be highlighted as illustrations, like the LP stuff (since linear programming is important to lots of people), and these could get one or two paragraphs instead of one or two sentences.

• where to get more information? Or is this well-enough explained already?