Fix some English errors (an GIT -> a git, installation instruction ->
installation instructions, etc).

Specify "console" as the language of all the shell code blocks, this
syntax highlights a shell prompt.

Markup filenames with italics.

Debian comes with sudo by default, omit the root shell installation
example and keep only the sudo one for brevity.

Make some of the text briefer, carefully making sure no instruction is
lost and no ambiguity is introduced.