FEAT(content): add some DS pages

...llact/demarches-simplifiees/support/instructeur/_index.md

...e-dois-confirmer-mon-compte-a-chaque-connexion/README.txt

...je-dois-confirmer-mon-compte-a-chaque-connexion/_index.md

...s-adullact/demarches-simplifiees/support/usager/_index.md

...arches-simplifiees/support/usager/erreur-siret/README.txt

...marches-simplifiees/support/usager/erreur-siret/_index.md

...ifiees/support/usager/je-ne-recois-pas-d-email/README.txt

...lifiees/support/usager/je-ne-recois-pas-d-email/_index.md