Add vignettes and pkgdown website

[marohrdanz]

This pull request adds vignettes and infrastructure for a pkgdown website.

A few things to mention:

• I updated the version number to 0.12.11 to reflect addition of vignettes
• GitHub Actions builds the pkgdown website from master/main branches, and deploys it to gh-pages branch at the root level
  • This is configured in .github/workflows/pkgdown.yaml
  • Must push to repo with personal access token with workflow scope for the build to work
• Probably worth squashing all the commits in this PR

If/After this PR is accepted, there are some steps to take to set up the pkgdown site:

1. Change the url in _pkgdown.yml file to https://MD-Anderson-Bioinformatics.github.io/NGCHM-R/

2. Change the GitHub href at the bottom of _pkgdown.yml to https://github.com/MD-Anderson-Bioinformatics/NGCHM-R/

3. Set up empty gh-pages branch. Example commands:

   git checkout --orphan gh-pages
   git rm -rf .
   git commit --allow-empty -m 'Initial gh-pages commit'
   git push origin gh-pages
   git checkout master

4. Configure GitHub Pages to use 'gh-pages' branch at the root level.

   • Follow GitHub's documentation for this

[marohrdanz]

Thanks @ChrisWakefield . I think the commits can be squashed during the merge.