Destroy history of github-pages branch (#3783)

## Motivation and Context

Keeping the history of `github-pages` is not useful, and it hogs the
output of `git log --graph --all` and other such views of the repository
history. In the event that someone would need to look at old contents of
the website, it can be regenerated using `mdbook` from any commit on
`main`.

<img
src="https://github.com/user-attachments/assets/f37630fa-92ef-4e49-a9e5-a1357a785e46"
width="550">

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

.github/workflows/github-pages.yml

@@ -34,17 +34,18 @@ jobs:
         git config --local user.name "AWS SDK Rust Bot"
         git config --local user.email "[email protected]"
         git remote set-url origin https://x-access-token:${GITHUB_TOKEN}@github.com/${{ github.repository }}
-        git fetch origin github-pages
+        REV=$(git rev-parse --short HEAD)
 
         pushd design &>/dev/null
         cargo install mdbook
         cargo install --locked mdbook-mermaid
         mdbook build --dest-dir ../../output
         popd &>/dev/null
 
-        git checkout origin/github-pages -b github-pages
-        rm -rf design
+        git checkout --orphan github-pages
+        git rm --cached -r .
+        git clean -ffdx
         mv ../output design
         git add design
-        git commit -m "Update design docs"
-        git push origin github-pages
+        git commit -m "Design docs @ ${{ github.repository }}@${REV}"
+        git push origin github-pages --force