Rebrand: 1.0 images #1028

Part of #1024. Step 4, needed by the docs and Discussions updates.

• 1.0 hero (docs/release-notes/images/1.0.0_hero*.png, docs/blog-posts/2026-10-release-1.0/images/1.0.0_hero*.png): the rebranded hero already exists in light and dark (same design as the documentdb.io post hero, Brand unification: DocumentDB extension for Visual Studio Code documentdb/documentdb.github.io#226). Swap it in and update the alt text.
• Road to 1.0 illustration (1.0.0_road_to_1.0_illustrated*.png, release notes and blog post): still shows the old name. Regenerate, edit or replace, light and dark.
• README and docs hero screenshot alt text (vscode-documentdb-hero-screenshot*.png): text only, image unchanged.
• Repository social preview image: use the rebranded hero (operator uploads it, see the post-release issue).