styleguide: Code consistency improvements

[hiwelo]

This pull request contains some code consistency improvements for configuration and template files used by the styleguide.

For the moment, these files contains a mix between HTML, Markdown and Jade. I try to improve the readability of these files by replacing all HTML by the appropriate Markdown for the styleguide content or Jade for the styleguide examples.

[jnoesser]

Merci beaucoup pour ton retour et cette remise au propre des fichiers, c'est bien plus clair ainsi. 👍

[PhilippeVay]

Thanks for clarifying the examples.

I noticed a couple of typos in this PR and a bunch of preexisting ones, will commit them in master directly.

• Small : s/em/small
• table Pays s/l/p
• Jade/Pug needs more indenting with input where a required asterisk appears <label for="a1"></label>Champ texte (placeholder) (really not intuitive here, meh)
• "Une grille multi-lignes" : empty div, will add "lorem" text
• new div in <section class="sg-block sg-section-votre-grille-avec-des-mixins-sass"> <div class="sg-text">. Not sure yet if it's OK or not.
• à la fin de Grid, les 2 "(pas vraiment) blocs" .grid-perso { @include grid(4, 0); } et .grid-perso { @include grid(4, 1rem); } sont ignorés (à re-inliner je suppose)