Adopt instinct-design flavor: Instinct Design System landing-page redesign

[AMD-melliott]

Motivation

Cut over instinct-docs to the official instinct-design flavor shipped by rocm-docs-core, and redesign the landing page and System Administrators subpages around a shared, reusable icon-card component. This replaces the earlier manual asset port and the older sphinx-design grid-item-card/photography-based cards with something more consistent and easier to maintain.

Technical Details

• Adopt flavor: "instinct-design" (superseding the manually-vendored design-system assets from the original port).
• Introduce system-admin/_cards/*.md — reusable MyST {include} partials, one per tool/resource card — used across the landing page and all System Administrators subpages (bare-metal, cluster, co-tools, cloud, virtualization) instead of duplicating card markup per page.
• Add a collapsible primary sidebar with a working toggle, a sticky on-page rail, and default-open System Administrator groups.
• Fix the dead docs/_static/css/index.css static-asset reference in conf.py's setup() (the real index.css was already loading correctly via html_css_files; the add_css_file call pointed at a path that never existed).
• Consolidate the separately-added docs/resources/{index,common-reference}.md pages into the redesign's existing landing-page sections, since their content (Enterprise AI, Omnistat, CVS, AMD SMI, RVS, Instinct Micro-architecture, HIP C++, OpenMP, etc.) was fully duplicated there.
• Pick up rocm_docs_generate_llms and the new repository_url/use_repository_button/use_issues_button/use_download_button theme options already on develop.
• Ported forward content added to develop since this branch diverged — Spur, MI3XX Reference Design, AMD-DriveNets System Reference Architecture, and a RoCE traffic-analysis blog card — into the new icon-card format so nothing added upstream in the interim was lost.

Test Plan

• Confirm the ReadTheDocs dev build for this PR succeeds with no Sphinx warnings/errors.
• Spot-check the landing page, System Administrators subpages, dark/light toggle, and sidebar toggle behavior.
• Confirm no dangling references to the removed docs/resources/* pages.

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.