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.

Some links are not valid as they are only on GitHub (not part of our
team), other links are not valid as some contributors have not logged
into our GitLab yet

 - Update to latest mdbook
   - Switch to using book.toml
   - No empty links are allowed anymore. Comment out all empty links in
     SUMMARY.md
 - Add theme
   - Add favicon.png

* Fix spelling: github -> GitHub, etc.
* Update the GitHub team list
* Move `null:` and `zero:` to the userspace schemes
* Change icons from Oxygen to Faba/Moka
* Remove statements that TFS is a ZFS implementation
* Add Intel HDA to the supported hardware
* Remove the `libredox` guideline
* Add some outgoing links where I think they're necessary

It's much better these days!

    $ cd redox
    $ git grep "unsafe" kernel/ | wc -l
    67
    $ loc kernel/
    --------------------------------------------------------------------------------
     Language             Files        Lines        Blank      Comment         Code
    --------------------------------------------------------------------------------
     Rust                    42         5743          894          366         4483

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`.

Fix typo