Prevent user-guide page cover to flash on page load

[Bonapara]

Current behavior

The flash is because the content is loaded before the image cover:

https://github.com/twentyhq/twenty/assets/19412894/62d64f09-4deb-4878-abdc-7ff073a927d1

Desired Behavior

Display a gray placeholder under the image to prevent the page from flashing. Make it appear smoothly when loaded (150ms fade in animation)