feat: [FC-86] implement index page config api

[andrii-hantkovskyi]

Add Index Page Config API

This PR introduces a new API endpoint to provide dynamic configuration for the Catalog index page:

• IndexPageConfigView: A Django REST Framework view at /branding/index that returns a JSON payload of configuration settings.
• Settings are resolved using configuration_helpers.get_value() with support for SiteConfiguration overrides and fallbacks to Django settings.

The following keys are returned:

• enable_course_sorting_by_start_date
• homepage_overlay_html
• show_partners
• show_homepage_promo_video
• homepage_course_max
• homepage_promo_video_youtube_id

Unit tests verify fallback behavior, site-specific overrides, and expected default values for all fields.

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[andrii-hantkovskyi]

Hi @diana-villalvazo-wgu, would you have a chance to review this PR when you have a moment?

[brian-smith-tcril]

Could you provide context as to why the new MFE would need each of these?

I would think that most of this could be handled via plugin slots instead.

[cmltaWt0]

Closing this due to changed implementation.

[PKulkoRaccoonGang]

Reopened at #37130