[REF] documentation: reorganize setup/install

Odoo setup/install documentation needed a re-organization and an update
to reflect the latest dependencies versions.
There was too much warnings and danger signs on this page.
Finally, the structure was not the same for each install method.

* As CentOs is actually too old to handle python3 dependencies for Odoo,
  rpm install method is explicitely mentioning Fedora 26.
* The "yum" package manager has been deprecated in Fedora and replaced
  with "dnf".
* As Debian and Ubuntu don't ship a python3-num2words package, the
  documentation must provide a hint on how to install the module using pip.
* The documentation was missing the virtualenv way of installing Odoo.
* Paragraphs mentioning configuration file were removed. The
  reference/cmdline page should be updated consequently.