Move docs builds onto a VitePress line that no longer drags in old esbuild

Dependabot flagged the VitePress 1.6.4 line because its Vite dependency brought in an
older esbuild. Upgrading to the current VitePress alpha moves the docs toolchain
to Vite 7 and esbuild 0.27 while keeping this repository's current docs config,
build scripts, and Pages workflow working unchanged.

Constraint: The published CLI runtime should stay untouched; this change is limited to docs tooling
Constraint: The repo Pages site must keep building with the existing VitePress config and GitHub Actions flow
Rejected: Stay on VitePress 1.6.4 and override transitive esbuild manually | keeps the repo on an older unsupported toolchain branch
Rejected: Hold the upgrade until VitePress 2 is stable | leaves the Dependabot issue in place despite a verified compatible upgrade path
Confidence: medium
Scope-risk: moderate
Reversibility: clean
Directive: Revisit this dependency once VitePress 2 exits alpha so the repo can move back to a stable release line without redoing the docs surface
Tested: npm ls vitepress vite esbuild; npm run build; npm test; npm run docs:build; npm audit
Not-tested: GitHub Actions CI run for this dependency-only branch

Author: Ring-wdr