Major documentation overhaul - Main vignette and core function updates

[dgkf]

• Updated documentation to roxygen2 7.0.2
• Rewrote the "Getting Started" vignette
  • Has clearer breakdown of steps with links to documentation
  • Included diagram of core workflow
  • Introduced topics with a single package first, before introducing the multi-package workflow
• Updated pkg_ref documentation
• Updated pkg_ref_cache documentation
• Updated assess documentation to include a catalog of included assessments

[elong0527]

Thanks for updating the documentation and Roxygen version.