doc/CodingGuidelines: URLs and paths as monospace

doc/CodingGuidelines: URLs and paths as monospace

The current documentation uses both quotes (italics) and backquotes
(monospace) to render URLs and pathnames, which is inconsistent.
Document a best practice in CodingGuidelines to help reduce
inconsistencies in the future.

We set the best practice to using backquotes, since:

* It is already an established practice. For exemple:

  $ git grep "'[^']/*[^']'" | wc -l
  206

  $ git grep '`[^`]/*[^`]`' | wc -l
  690

  There are false positives on both sides, but after a cursory look at
  the output of both, it doesn't seem the false positive rate is really
  higher in the second case.

  At least, this shows that the existing documentation uses
  inconsistent formatting, and that it would be good to do
  something about it.

* It may be debatable whether path names need to be typed in
  monospace but having them in italics is really unusual.

Signed-off-by: Corentin BOMPARD <corentin.bompard@etu.univ-lyon1.fr>
Signed-off-by: Nathan BERBEZIER <nathan.berbezier@etu.univ-lyon1.fr>
Signed-off-by: Pablo CHABANNE <pablo.chabanne@etu.univ-lyon1.fr>
Signed-off-by: Matthieu MOY <matthieu.moy@univ-lyon1.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>