Add Artisan Build sponsor across site

[simonhamp]

What

Adds Artisan Build as a sponsor (a standard logo-only sponsor — not a Partner) across the site.

• Home page — new logo-only Sponsors section (<x-home.sponsors />) below the Partners section. Bare logos (no card, no description), a subtle "Sponsored by" label, and a "Become a sponsor" link to /sponsor. Built from a small $sponsors array so more can be added easily. Spacing (my-16 lg:my-24) separates it from the Partners and "What developers are saying" sections.
• Docs & blog sidebars (desktop + mobile) — new logo-only sponsor rotation (<x-sponsors.lists.docs.sponsors />), shown as a separate section from the existing Partner rotation, with no card and no description, plus a "Become a sponsor" link. The existing Partner blocks were relabeled in comments for clarity (they render Partners, not sponsors).
• The Vibes — Artisan Build added to the existing "Sponsored By" logo grid.

Why

Artisan Build is a sponsor, not a Partner, so they should appear as a plain logo with a link rather than in the richer Partner cards (which carry taglines and descriptions).

Assets

• public/img/sponsors/artisan-build.webp — dark-text logo for light mode
• public/img/sponsors/artisan-build-dark.webp — white logo for dark mode

Both link to https://artisan.build/ with NativePHP UTM parameters.

Notes

• The non-Partner sponsor list is currently inlined in two places (home section + docs/blog rotation), mirroring how the Partner list is already duplicated in the codebase. Happy to centralize into a config/sponsors.php if preferred.