menu: improve the menu

* Uses a regular method to generate the menu (`config.toml`).
* Improves the test to provide the good atom link according to the
  homepage, a section or a post.
* Provides a title to improve identification of the atom link.
* Adapts the menu style to mobile or wider screens.
* Adds and uses a mixin for inline lists.

Co-Authored-by: iGor milhit <igor@milhit.ch>