doc: add HOWTO_REBASE_ORIGINAL_REPO.md

HOWTO_REBASE_ORIGINAL_REPO.md

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+# How to rebase an original repository into this fork
+
+## Clone the original repository and create a new rebased branch
+
+```bash
+# Clone the original repository
+cd ../
+git clone https://codeberg.org/ziglings/exercises.git
+cd exercises
+
+# Set the remote to your fork
+git remote -v # should show origin https://codeberg.org/ziglings/exercises.git
+git remote set-url origin https://github.com/s-0-a-r/ziglings-exercises.git
+git remote -v # should show origin https://github.com/s-0-a-r/ziglings-exercises.git
+
+# Create a new branch from the original branch
+git switch -c original-branch
+
+# Push the branch to your fork
+git push -f origin original-branch
+
+# Switch back to your branch
+cd ../ziglings-exercises
+git fetch
+git switch -c rebase-original-branch
+
+# Rebase the original branch into your branch
+git rebase -i original-branch
+
+# Push the rebased branch to your fork
+git push -u origin rebase-original-branch
+```
+
+## Create a pull request
+
+Please create a pull request from your rebased branch to main.
+
+## Merge the pull request
+
+After the pull request is merged, you can delete the branch from this fork.