* Move overview and introduction files into a single folder
* Remove about_this_book.md We can add a guide to contributing to documentation,
  and mention the book there.
* Remove heartbleed case study This has been a stubbed out article for ~3 years.
  We might consider an article like this for a technical white paper,
  or a blog post, if someone gets inspired.
* Stub out design/drivers and design/memory files
* Remove will redox replace linux
* Differentiate: Philosophy, Why a New OS, Goals

Refs #134

The redox book contains some broken links throughout the various
markdown pages. Fix all broken links to existing pages (by using
relative paths only) such that the documentation can be used as
intended.

This reverts merge request !143

The redox book contains some broken links throughout the various
markdown pages. Fix all broken links (to existing pages) such that
the documentation can be used as intended.

The changes include:
- Editing for flow and understandability.
- Unifying how links are included in the source files.
- Adding an `About this Book` page to the introduction.
- Adding a few examples or new pieces of information I felt were
  lacking when I read the book.
- Unifying 'user space' and 'kernel space' spelling.
- Added myself to the authors list in `overview/welcome.md`.