Improve French installation page

 * Added missing comma
 * Removed reference to OCaml version
 * Translated comments to French
 * Fix Markdown formatting
Merge pull request #511 from pquentin/master

Improve French installation page
Merge pull request #512 from dsheets/opam.ocaml.org

opam.ocaml.org integration
opam.ocaml.org: link script and factor out opam2web style

cd opam.ocaml.org && ./link_blog_doc_ext.sh blog doc ext
Styles that every opam2web site will need are now in opam2web.css in that project.