remove the need for main.inc from the dev manual

Hello,

there is no need to have a main.inc anymore

https://fusiondirectory-developer-documentation.readthedocs.io/en/1.4/pluginsystem/start.html?highlight=main.inc#displaying-the-plugin-in-the-my-account-menu

The image should also be updated

Reference

main.inc files should not be needed. https://gitlab.fusiondirectory.org/fusiondirectory/fd/-/issues/5951