feat: add is_epi_archive

[dshemetov]

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}

[brookslogan]

Previously, we had is_epi_archive with a something like a grouped_okay arg deciding whether to count grouped archives or not. Does that sound useful?

Pre-merge-wise I just need to sanity-check the docs output. Can't tell what to expect from skimming the diff alone.

[brookslogan]

Spiffed up the docs a bit and put an (old) idea into #649. Think this is good to merge although I might wait a tiny bit if you feel like double-checking the docs change.

[dshemetov]

/preview-docs

[github-actions]

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