Book link fixes

See merge request !144

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 (by using
relative paths only) such that the documentation can be used as
intended.

This reverts merge request !143

Fix markdown book links

See merge request redox-os/book!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.

Fix broken link to 'Compiling Redox' chapter

See merge request redox-os/book!142

Fix external project URLs

See merge request !140

gcc7 is needed instead of gcc49

See merge request !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 redox-os/book!134

Fix redox releases link

See merge request redox-os/book!133

Update links to point to GitLab where possible

See merge request redox-os/book!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