Automatically create _sidebar or _sidebar.md when generating markdown output

[xg5-simon]

When generating markdown output for Atomic Threat Coverage create a basic _sidebar.md or _sidebar file to build wiki navigation. Wiki _sidebar navigation is supported by Gitlab.com and Github.com. Navigation items could be as follows:

1. Home (Top level page to include a description of Atomic Threat Coverage and links Data_Needed, Detection_Rules,...,Triggers
2. Data_Needed
3. Detection_Rules
   ...
4. Triggers