Delete extra files from the 'gh-pages' branch on build.

- This repo uses GitHub Actions and has a CI/CD process that builds the web site from the `master` branch and pushes all the built files to the `gh-pages` branch from where GitHub Pages deploys the site.
- Check [`./.github/workflows/build-website.yml`](https://github.com/HarshKapadia2/git_basics/blob/master/.github/workflows/build-website.yml) for the exact working of the CI/CD process.
- If files are renamed in the `master` branch and the CI/CD process is run, the original file and the new renamed file are both found in the `gh-pages` branch.
- The problem to be solved is that the original file should not be present in the `gh-pages` branch after the CI/CD process runs.
- One solution could be to delete all the files in the `gh-pages` branch before the newly built files are pushed into the branch.
- Please note that, I **do not** want the entire `gh-pages` branch to be deleted and created every time the CI/CD process runs, as I want to have a history of the previous versions of the site.
- Do let me know if you have any other solution in mind.