Switch DIRECTORY-STRUCTURE.md to directories

[stockholmux]

In the DIRECTORY-STRUCTURE.md it was articulated that content should go into directories with named markdown files. While this assumption is fine for hugo by itself but the docsy template has some issues rending a proper sidebar using this method. As an example of the improper rending
/os/1.13.x/concepts/updating-bottlerocket.markdown would generate a sidebar listing "Updating Bottlerocket" at both the 1.13.x level and at the concepts level in the sidebar.

If we move to having each piece of content having it's own subdirectory (e.g. /os/1.13.x/concepts/updating-bottlerocket/) and the content itself going into a index.markdown file, Docsy's sidebar renders properly. (This change in the content was made in #38)

As a result, the DIRECTORY-STRUCTURE.md file should be updated to reflect this reality. So, all foo.markdown files should have the .markdown removed except where it's specifically needed.