Update branding documentation to reflect changes fixes done on MTM-65244

[carlosceia]

Updated branding documentation across 4 files to emphasize CSS variables with the branding editor as the recommended approach, deprecated brandingEntry, and added Angular 18 migration guidance.

Changes

branding-your-application.md

• Split documentation into "Approach 1: Recommended" (CSS variables) and "Approach 2: Advanced" (LESS variables)
• Added benefits comparison (no rebuild, user-friendly UI, tenant-wide, dynamic changes)
• Added complete setup steps with numbered instructions
• Removed deprecated brandingEntry references
• Emphasized branding editor as primary customization method

application-options.md

• Removed brandingEntry from flavor configuration example
• Added proper angular.json configuration for flavor-specific branding
• Documented how to use different branding files per build configuration

upgrading-to-angular-18.md

• Added complete brandingEntry deprecation notice
• Included migration steps with code examples (create styles.less, configure angular.json)
• Added important notes about import order for extend.less

c8y-cli.md

• Deprecated --app.brandingEntry CLI flag
• Added migration guidance to use Angular CLI styles configuration instead

Impact

• ✅ Clear guidance on recommended (CSS) vs advanced (LESS) approaches
• ✅ Smoother Angular 18 migration path for customers
• ✅ Consistent messaging across all branding documentation
• ✅ Better user experience for non-developer administrators
• ✅ No breaking changes - existing implementations continue to work

Related

This complements the technical fixes in the cumulocity-ui repository (https://github.com/Cumulocity-IoT/cumulocity-ui/pull/10345) that enable LESS variable customization to work correctly.

[github-actions]

Preview available here

[BeateRixen]

Suggested change

	**Import order is critical:** Always import `@c8y/style/extend.less` **FIRST** at the top of your file, then override variables AFTER. This ensures the conditional guard system can detect your overrides correctly.
	**Import order is critical:** Always import `@c8y/style/extend.less` **first** at the top of your file, then override variables **after**. This ensures the conditional guard system can detect your overrides correctly.

[BeateRixen]

I adjusted the highlighting in several cases. One method, either all caps or bold, is enough :-).