Reorganise Concepts folder

This PR:
1) Moves the topics out of the folders they were previously in
   from the operator-registry folder. This fixes the issue of these
   topics being highlighted under Operator-registry heading, unlike
   other topics under other headings that are not highlighted.
2) Puts the dependency-resolution doc inside it's own folder to
  highlight the topic of dependency-resolution in the site.

Author: anik120