Is it obligatory to withdraw from a project to establish a website based on user contributions, given the possibility of it violating intellectual property rights, and does sin accrue from participating in its foundation after discovering that?

If the website's purpose is to enrich knowledge and disseminate beneficial sciences, then there is no sin upon you for what is published without your control. The sin falls upon those who infringe upon the rights of others, and your warning suffices. Quoting information from books and websites is permissible and not an infringement on intellectual property, provided that academic integrity is observed and statements are attributed to their sources. The author's permission is not required for نقل (transcription/transmission) after publication. The fundamental rule is academic integrity, and it is imperative to spread a culture of respecting people's rights and their legitimate Islamic regulations.