fix(ci): install the docs-pinned pnpm instead of floating on 11.x

pnpm/action-setup resolved 'version: 11' to the newest 11.x, and its self-installer crashes upgrading to 11.12.0 (Cannot use 'in' operator to search for 'integrity'), failing both docs workflows at setup. Reading docs/package.json instead installs the exact packageManager pin (pnpm@11.9.0), which also keeps the workflows and the lockfile toolchain on a single source of truth.

.github/workflows/docs-ci.yml

@@ -26,7 +26,7 @@ jobs:
 
       - uses: pnpm/action-setup@v6
         with:
-          version: 11
+          package_json_file: docs/package.json
 
       - uses: actions/setup-node@v6
         with:

.github/workflows/docs-deploy.yml

@@ -35,7 +35,7 @@ jobs:
       - uses: actions/checkout@v7
       - uses: pnpm/action-setup@v6
         with:
-          version: 11
+          package_json_file: docs/package.json
       - uses: actions/setup-node@v6
         with:
           node-version-file: .nvmrc