doc: documented images organisation and refactored code

static/img/components/HomepageFeatures/hello.png

static/img/components/HomepageFeatures/rainbow.svg

static/img/components/HomepageFeatures/read-smile.svg

static/img/components/HomepageFeatures/selfie.svg

static/img/components/HomepagePromises/bouclier.svg

static/img/components/HomepagePromises/liberte.svg

static/img/components/HomepagePromises/rgpd.svg

.../img/docs/gestions_des_plans/My_Froggit_Customer_Page.png

.../docs/gestions_des_plans/customer_group_in_group_list.png

...mg/docs/gestions_des_plans/froggit_data_in_group_list.png

static/img/docs/gestions_des_plans/invite_members_button.png

static/img/docs/gestions_des_plans/invite_members_form.png

static/img/docs/gestions_des_plans/invite_members_mail.png

...ic/img/docs/gestions_des_plans/invite_members_pending.png

static/img/docs/gestions_des_plans/invite_members_result.png

static/img/docs/gestions_des_plans/remove_members_button.png

static/img/pages/communaute/cheatsheet_cover.png

static/img/pages/tarifs/cauldron.png

static/img/pages/tarifs/computer.png

static/img/pages/tarifs/hidden.png