Setup pkgdown so it hosts release and dev sites

[gvegayon]

This pull request includes several changes to the GitHub Actions workflow for building the pkgdown site, as well as minor updates to the project configuration files. The most important change sets two version of the pkgdown site: release (root) and dev (nested).

Workflow Improvements:

• .github/workflows/pkgdown.yaml: Added matrix builds for 'release' and 'devel' R versions to the pkgdown site build job.
• .github/workflows/pkgdown.yaml: Improved caching mechanism to check if the release is cached and skip unnecessary steps if the cache is hit.
• .github/workflows/pkgdown.yaml: Modified the workflow to combine artifacts from both 'release' and 'devel' builds before deployment. [1] [2]

Configuration Updates:

• DESCRIPTION: Updated the package version from 0.1.0 to 0.1.0.99 to indicate a development version.
• _pkgdown.yml: Enhanced the pkgdown configuration to include a development mode and added links to release and devel versions of the site.

[github-actions[bot]]

Thank you for your contribution, @gvegayon :rocket:! Your page is ready to preview here

[gvegayon]

The latest commit features the discussed changes, @kaitejohnson. I added a link to the top menu that allows switching between versions of the site. Notice the links will not work as expected since those are pointing to the absolute URLs. Here is teh releant artifact: https://github.com/CDCgov/ww-inference-model/actions/runs/11298508809/artifacts/2047035333.

[kaitejohnson]

I just added pkgdown site build (devel) and pkgdown site build (release).

The site looks great to me, thanks @gvegayon! I might wait until Tuesday to merge just in case anything happens that I don't understand with the deployment...

[gvegayon]

I love my font-awesome icon to switch between versions :P