Document optgroup for backend forms dropdown field

[mjauvin]

Depends on: wintercms/winter#1427

Summary by CodeRabbit

• Documentation
  • Added examples for grouped dropdown options, including how to structure groups and indicate icons for items.
  • Updated theme documentation with formatting and wording refinements for clearer section separation, consistent headings/labels, and reset of enumerated step sequences; no behavioral or functional changes.

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[coderabbitai]

Walkthrough

Adds documentation for option grouping (optgroup) in backend/forms.md with a multidimensional YAML example and note about icons; applies formatting and wording tweaks to cms/themes.md (blank-line and punctuation/numbering adjustments) without changing behavior.

Changes

Cohort / File(s)	Summary
Dropdown option grouping docs backend/forms.md	Introduces examples and guidance for option grouping (optgroup) in dropdown fields using a multidimensional YAML structure; notes that grouped items may include icons/images.
Theme docs formatting cms/themes.md	Formatting and phrasing adjustments (blank lines near headings/lists, punctuation and capitalization normalization, renumbered steps) with no functional or API changes.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

• Files are documentation-only; review for wording/format fidelity in backend/forms.md and visual/numbering consistency in cms/themes.md.

Poem

🐰 I hopped through YAML, tidy and spry,
Grouped options stacked, icons nearby,
A nibble of docs, polished and bright,
Dropdowns now ordered, snug and right,
Hooray — I thumped once, then took a bite! 🥕

Pre-merge checks and finishing touches

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Document optgroup for backend forms dropdown field' is directly related to the main change in the PR, which adds documentation for option grouping (optgroup) in backend forms dropdown options.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch dropdown-optgroup

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