Enable `keep_files` in GitHub Pages workflow

[seanprashad]

See https://github.com/peaceiris/actions-gh-pages#%EF%B8%8F-keeping-existing-files-keep_files - we need this because the directory with all of the content is being removed in the current Workflows:

Prepare publishing assets
  [INFO] ForceOrphan: false
  /usr/bin/git clone --depth=1 --single-branch --branch gh-pages ***github.com/seanprashad/leetcode-patterns.git /home/runner/actions_github_pages_1659116993419
  Cloning into '/home/runner/actions_github_pages_1659116993419'...
  [INFO] clean up /home/runner/actions_github_pages_1659116993419
  [INFO] chdir /home/runner/actions_github_pages_1659116993419
  /usr/bin/git rm -r --ignore-unmatch *
  rm 'asset-manifest.json'
  rm 'assets/icons/Adobe.png'
  rm 'assets/icons/Airbnb.png'
  rm 'assets/icons/Akuna Capital.png'
  rm 'assets/icons/Amazon.png'
  rm 'assets/icons/Apple.png'
  rm 'assets/icons/Asana.png'
  rm 'assets/icons/Atlassian.png'
  rm 'assets/icons/BlackRock.png'
  rm 'assets/icons/Bloomberg.png'
  rm 'assets/icons/Capital One.png'
  rm 'assets/icons/Citadel.png'
  rm 'assets/icons/Facebook.png'
  rm 'assets/icons/Goldman Sachs.png'
  rm 'assets/icons/Google.png'
  rm 'assets/icons/Intuit.png'
  rm 'assets/icons/JPMorgan.png'
  rm 'assets/icons/LinkedIn.png'
  rm 'assets/icons/Lyft.png'
  rm 'assets/icons/Microsoft.png'
  rm 'assets/icons/Morgan Stanley.png'
  rm 'assets/icons/Oracle.png'
  rm 'assets/icons/Palantir.png'
  rm 'assets/icons/Pinterest.png'
  rm 'assets/icons/Qualtrics.png'
  rm 'assets/icons/Quora.png'
  rm 'assets/icons/Salesforce.png'
  rm 'assets/icons/Snapchat.png'
  rm 'assets/icons/Square.png'
  rm 'assets/icons/Tesla.png'
  rm 'assets/icons/Twitch.png'
  rm 'assets/icons/Twitter.png'
  rm 'assets/icons/Two Sigma.png'
  rm 'assets/icons/Uber.png'
  rm 'assets/icons/Yelp.png'
  rm 'assets/images/Blind.png'
  rm 'assets/images/Educative.png'
  rm 'assets/images/Hackernoon.png'
  rm 'favicon.ico'
  rm 'index.html'
  rm 'manifest.json'
  rm 'robots.txt'
  rm 'static/css/2.5bd0ccc4.chunk.css'
  rm 'static/css/2.5bd0ccc4.chunk.css.map'
  rm 'static/css/main.fcf70d57.chunk.css'
  rm 'static/css/main.fcf70d57.chunk.css.map'
  rm 'static/js/2.6233fcec.chunk.js'
  rm 'static/js/2.6233fcec.chunk.js.LICENSE.txt'
  rm 'static/js/2.6233fcec.chunk.js.map'
  rm 'static/js/main.e97e6adf.chunk.js'
  rm 'static/js/main.e97e6adf.chunk.js.map'
  rm 'static/js/runtime-main.9a6761f7.js'
  rm 'static/js/runtime-main.9a6761f7.js.map'
  [INFO] chdir /home/runner/actions_github_pages_1659116993[41](https://github.com/seanprashad/leetcode-patterns/runs/7583614858?check_suite_focus=true#step:6:44)9
  [INFO] prepare publishing assets
  [INFO] copy /home/runner/work/leetcode-patterns/leetcode-patterns/build to /home/runner/actions_github_pages_16[59](https://github.com/seanprashad/leetcode-patterns/runs/7583614858?check_suite_focus=true#step:6:62)116993419
  cp: no such file or directory: /home/runner/work/leetcode-patterns/leetcode-patterns/build/.*
  [INFO] delete excluded assets
  rm: no paths given
  [INFO] Created /home/runner/actions_github_pages_1[65](https://github.com/seanprashad/leetcode-patterns/runs/7583614858?check_suite_focus=true#step:6:68)911[69](https://github.com/seanprashad/leetcode-patterns/runs/7583614858?check_suite_focus=true#step:6:72)[93](https://github.com/seanprashad/leetcode-patterns/runs/7583614858?check_suite_focus=true#step:6:96)419/.nojekyll