MMS (Multisig Messaging System) manual added to user guides

_i18n/ar.yml

_i18n/ar/resources/user-guides/multisig-messaging-system.md

_i18n/de.yml

_i18n/de/resources/user-guides/multisig-messaging-system.md

_i18n/en.yml

_i18n/en/resources/user-guides/multisig-messaging-system.md

_i18n/es.yml

_i18n/es/resources/user-guides/multisig-messaging-system.md

_i18n/fr.yml

_i18n/fr/resources/user-guides/multisig-messaging-system.md

_i18n/it.yml

_i18n/it/resources/user-guides/multisig-messaging-system.md

_i18n/pl.yml

_i18n/pl/resources/user-guides/multisig-messaging-system.md

_i18n/ru.yml

_i18n/ru/resources/user-guides/multisig-messaging-system.md

resources/user-guides/index.md

resources/user-guides/multisig-messaging-system.md