react-native-firebase

docs(messaging): add instructions for images in ios notifications (#4225)

* docs(messaging, notifications-with-image): add instruction for Notification Extension
* docs(messaging, notifications): add Podfile to spellcheck
* docs(messaging, notifications): add newly created Notifications with Image page to the sidebar
* chore(images): add assets folder for docs' images
* chore(images): moving images into the 'static' folder
* docs(messaging): reference images in the static folder

Co-authored-by: Mike Hardy <github@mikehardy.net>