This reverts merge request !143

src/explore/explore.md contains a link to a "Utilities" file. This file
no longer exists (it was removed in commit 2854c9b0). In order to avoid
unnecessary 404-type errors, remove the link to this page.

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.

Looking at `homebrew-gcc_cross_compilers` at homebrew-gcc_cross_compilers!4 (062060c4), 
it looks like gcc7 is needed and not gcc49.

Signed-off-by: Jérôme Jutteau <jerome@jutteau.fr>

Changes instances of
>an URL
to
>a URL

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

I also disambiguated the "Exploring Redox" title that was shared between
a page and a chapter

To help users who haven't previously installed software via cargo

loads -> load

 - 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

 - Update the asking questions page
 - Update the side projects page

* 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

I didn't have cmake installed on arch, and my build failed. Thought it should be included in the book

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

"Null-terminated" is the more common term, and I think better suited to denote that the string (being the object) is being terminated by a null. Feel free to edit my commit in that regard, if you prefer the current ending.

"a URL…" being singular.