This is an example from stackoverlow question.

cd path/to/project-b
git remote add project-a /path/to/project-a
git fetch project-a --tags
git merge --allow-unrelated-histories project-a/master # or whichever branch you want to merge
git remote remove project-a

Based on the above model you already have your project, my-project.

And you wish to merge git@github.com:tomrake/my-project-skel.git

cd path/to/my-project
git remote add doc-site git@github.com:tomrake/my-project-skel.git
git fetch doc-site --tags
git merge --allow-unrelated-histories doc-site/main # I use the main branch
git remote remove doc-site