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">

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