docs, sync compiler matrix in docs/index.md with README.md

[steven-varga]

The compiler/build matrix in docs/index.md is out of sync with README.md:

• Different compiler columns: docs has Clang 14/15/17; README has Clang 17/18/19/20 + Apple Clang + MSVC.
• Different OS labels: docs uses generic macOS and Windows; README uses macOS 15 and Windows.
• Missing NA cells: docs fills every cell with a badge; README correctly marks unavailable combos as NA.
• Stale badge URLs: docs points to vargaconsulting.github.io; README points to vargalabs.github.io.
• Missing top badges: docs/index.md is missing codecov, DOI, release, docs, and download badges.

Expected: the docs front-page matrix and badge URLs should match the README exactly so users see consistent CI status regardless of which page they land on.