* 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

Since Bell Labs has died, the Plan9 project website only exists at a
mirror on 9p.io

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`.

You asked for nitpicking, so here I go :). BFDL should only be used for
large projects that have been around for a while and have an established
leader who no one could every see changing. The side projects are likely
to have their maintainers change and are not so large as to warrant
calling anyone a BFDL.

You took some wiki content so I am taking some credit: :-P