docs: reference rc.9 and v1.0.0 release dates

[johnleider]

Updates docs and the v1.0.0 GitHub milestone to reference the scheduled release window:

• rc.9 — final release candidate, 2026-07-21 (Tue)
• v1.0.0 — stable release, 2026-07-22 (Wed)

Changes

• roadmap.md — DocsTimeline: v1 date Q3 2026 → July 22, 2026; rc entry + intro prose note the final rc.9 cut on July 21.
• AppBanner.vue — site banner now announces the July 22 stable release instead of the generic "now a release candidate".
• why-vuetify0.md — Road-to-v1 line (Q3 2026) → (July 22, 2026).
• README.md (packages/0, root symlink) — @rc install note + contributing section reference the July 22 stable date.
• guide/tooling/vuetify-cli.md — example version 1.0.0-rc.8 → 1.0.0-rc.9.

Also updated out-of-band (not in this diff): the v1.0.0 GitHub milestone description now reads "stable release scheduled 2026-07-22" with the rc.9/July-21 detail — this drives the "Now" block of the docs roadmap live via the GitHub milestones API.

All copy is future-tense/scheduled (today is 2026-07-16); nothing claims the stable release has shipped.

Not touched (flagged, out of scope): constants/releases.ts RELEASE_ALIASES (no rc/1.0.0 alias — renders raw, fine); constants.md example version string; devkey.md historical alpha framing.