Fix external project URLs

See merge request redox-os/book!140

gcc7 is needed instead of gcc49

See merge request redox-os/book!139

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

Fix broken SVG path in boot process

See merge request redox-os/book!138

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

Grammar cleanup

See merge request redox-os/book!137

Changes instances of
>an URL
to
>a URL

Update linux dependencies

See merge request redox-os/book!136

Fix permission denied on libc git clone

See merge request redox-os/book!135

Rewrite some links to HTTPS

See merge request !134

Fix redox releases link

See merge request redox-os/book!133

Update links to point to GitLab where possible

See merge request !132

Add more detail on Sodium

See merge request redox-os/book!131

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

add packages needed for build

Gave a hint about adding xargo to the path

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

added login information

Fixed typo in Real Hardware chapter

loads -> load

Massive mdbook update

updated instalation instructions for rusthello

fixed broken link to Enricos github

Fix "preparing the build" chapter linking back to a previous chapter as next step

Clear up the documentation for setting up and compiling

 - 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