replace old app links with links in catalog

[xxczxp]

since some of the independent app repositories like https://github.com/unikraft/app-helloworld are no longer maintained, is it need to replace these links in doc with the catalog repository's link?

[StefanJum]

Thank you @xxczxp @pwhMass. Please sign-off the commit, and use the naming schema that is used in the rest of the repository as well (something like docs: Replace old app links with links in catalog), then we can merge this.

[razvand]

@xxczxp , could you please follow @StefanJum 's comments to improve the commit message?