Introduce "subchapters" or "tours"

[wilzbach]

As said in #265, I am concerned since the GA Analytics results that those our categories are:

• too wide (common rule is to have at most 7 items)
• put the "best parts" at the end

My best ideas currently are

• introduce subchapters and thus have grouping in the section like (Object oriented programming, control flow, ...
• introduce "tours" and reuse the existing hiearchy (e.g. "Basics" would be an entire tour)
• instead of "Basics" and "Gems" split into more chapters

Related to this idea is #52 (menu at the left side).

[stonemaster]

Thinking about it I would prefer having #52 implemented like in the go tour and have more chapters. Basics could be split in Basics and Advanced. And Gems could be split up into two other topics.
I think that subsections might make navigation more difficult. At the end the tour should be just a "quick" tour on the D language and we shouldn't start competing with such great work we already have in form of D related books.

[wilzbach]

This issue was moved to dlang-tour/english#48