Add color management section for calendar

[Misiu]

Proposed change

Describe color customization for calendar entities via entity registry as suggested by home-assistant/architecture#883 (reply in thread)

Type of change

• Document existing features within Home Assistant
• Document new or changing features for which there is an existing pull request elsewhere
• Spelling or grammatical corrections, or rewording for improved clarity
• Changes to the backend of this documentation
• Remove stale or deprecated documentation

Checklist

• I have read and followed the documentation guidelines.
• I have verified that my changes render correctly in the documentation.

Additional information

• This PR fixes or closes issue: fixes #
• Link to relevant existing code or pull request: Add color setting for calendar entities frontend#28882, Add initial_color property to CalendarEntity core#145606

Summary by CodeRabbit

• Documentation
  • Added a "Color management" section to the calendar docs explaining the new initial color setting (hex string), how the default color is stored and read by the frontend, and how users can customize calendar colors via settings; includes usage examples and notes on when defaults apply.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Adds a "Color management" section to calendar documentation describing that an initial_color hex string can be provided, stored in entity registry options when the entity is first added, read by the frontend, and set via get_initial_entity_options with a Python example.

Changes

Cohort / File(s)	Summary
Documentation docs/core/entity/calendar.md	Added an initial_color property to the Calendar entity properties and a "Color management" section explaining: how to provide an initial_color hex string, that it is stored in entity registry options when first added, how the frontend reads it, and how to supply a default via get_initial_entity_options with a Python example.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly and clearly describes the main change: adding a color management section to the calendar documentation.
Description check	✅ Passed	The description follows the template structure with all required sections completed, including proposed change, type of change, checklist items checked, and relevant pull request links provided.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[home-assistant]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks 👍

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