Restructure "customise the CMS" docs

Overview

The current docs at https://github.com/silverstripe/silverstripe-framework/tree/master/docs/en/02_Developer_Guides/15_Customising_the_Admin_Interface are a hodge podge of legacy and new documentation. While some docs are marked as "deprecated", there's no clear learning path for new features.

Acceptance Criteria

• As a developer customising the CMS, I can follow a clear learning path for React-based CMS sections without getting confused by legacy usage
• I can get a quick overview of the means to customise the CMS without diving into details
• I have a path for setting up a SilverStripe CMS module with frontend tooling (either through docs or a bare bones code repository)
• Legacy documentation is clearly marked on overview and navigational pages
• Where my customisation my involve multiple approaches (e.g. a reducer and new component), I can easily find relevant sections through cross-linking
• Legacy docs are stripped to their essentials, with minimal outdated advice (e.g. "we test with JUnit")

Start with the existing docs under docs/en/02_Developer_Guides/15_Customising_the_Admin_Interface and compare the legacy and React-based sections against the acceptance criteria. Map the overview, navigation, setup guidance, and cross-links so a developer can follow a clear CMS customisation path, while clearly marking legacy material and removing outdated advice.