Lcb/wip vignettes demos dogfooding

[brookslogan]

Checklist

Please:

• Make sure this PR is against "dev", not "main" (unless this is a release
  PR).
• Request a review from one of the current main reviewers:
  brookslogan, nmdefries.
• Makes sure to bump the version number in DESCRIPTION. Always increment
  the patch version number (the third number), unless you are making a
  release PR from dev to main, in which case increment the minor version
  number (the second number).
• Describe changes made in NEWS.md, making sure breaking changes
  (backwards-incompatible changes to the documented interface) are noted.
  Collect the changes under the next release number (e.g. if you are on
  1.7.2, then write your changes under the 1.8 heading).
• See DEVELOPMENT.md for more information on the development
  process.

Change explanations for reviewer

Magic GitHub syntax to mark associated Issue(s) as resolved when this is merged into the default branch

• Resolves #{issue number}

[dsweber2]

closes #355

[brookslogan]

• Re. backtesting: We have https://cmu-delphi.github.io/epipredict/articles/backtesting.html to point to, and with Handle version datetime or different resolution than time_value #653 some of the other discussion should be unnecessary. Though some of the archive manipulation patterns in the epipredict vignette make me a bit uncomfortable, but these are due to epiprocess shortcomings (lack of as_tibble(), dplyr/tidyr verbs, and a way to not lose info about implicit vs. explicit NAs from the archive format, during merges especially).
• Latency part... is why I'm not just closing this. Might need updated now that there is revision_summary(), though revision_summary() examines things from a slightly different perspective. And there is plotting code we might want to still present or incorporate into functions.