Define and document repository structure

[sdruskat]

Moved here from https://github.com/hexatomic/meetings/issues/1

Decisions (based on discussion during the 1st project meeting and following):

Decisions:

1. All software-related documentation sources go into gh.com/hexatomic/hexatomic/docs folder: user docs, dev docs, maintainer docs
2. All rendered documentation (user, dev, maintainer) are deployed to hexatomic.github.io/hexatomic/{minor-version}/{user|dev|maintainer}
3. All project-related documentation goes into the h.gh.io repo, and succinct subfolders: project management, etc.
4. All project-related artifacts, e.g., project management files, etc., go into the h.gh.io src branch.

• Document this in maintainer documentation.
• Document this in the project documentation as a design decision.