Document image group usage for sky matching

[mcara]

Resolves JP-3843
Fixes #9063

This PR adds description of image groups in relation to sky matching and when grouping is desirable or not.

Tasks

• request a review from someone specific, to avoid making the maintainers review every PR
• add a build milestone, i.e. Build 11.3 (use the latest build if not sure)
• Does this PR change user-facing code / API? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types)
  • update or add relevant tests
  • update relevant docstrings and / or docs/ page
  • start a regression test and include a link to the running job (click here for instructions)
    • Do truth files need to be updated ("okified")?
      • after the reviewer has approved these changes, run okify_regtests to update the truth files
• if a JIRA ticket exists, make sure it is resolved properly

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9080   +/-   ##
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  Coverage        ?   76.94%           
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  Files           ?      498           
  Lines           ?    45844           
  Branches        ?        0           
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  Hits            ?    35273           
  Misses          ?    10571           
  Partials        ?        0           

Flag	Coverage Δ		*Carryforward flag
nightly	77.38% <ø> (?)		Carriedforward from 5ce2717

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[emolter]

This looks fantastic Mihai. I just added a few comments to improve grammar/clarity slightly.