Move Docusaurus docs site into docs/ subdirectory

Relocates the entire Docusaurus site from the repo root into a
dedicated docs/ directory to keep docs tooling separate from the
Rust/frontend workspace. Updates CI workflows (build.yml, docs.yml),
justfile (adds `just docs` task), and GEMINI.md to reflect the new
paths.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/build.yml

@@ -77,7 +77,7 @@ jobs:
             bun-docs-${{ runner.os }}-
 
       - name: Install docs dependencies
-        run: bun install --frozen-lockfile
+        run: cd docs && bun install --frozen-lockfile
 
       - name: Read VERSION
         id: ver
@@ -87,15 +87,15 @@ jobs:
           echo "version=${base}${{ inputs.version_suffix }}" >> "$GITHUB_OUTPUT"
 
       - name: Build docs site
-        run: bun run build
+        run: cd docs && bun run build
 
       - name: Package docs site
         shell: bash
         run: |
           set -euo pipefail
           ARCHIVE="alchemist-website-${{ steps.ver.outputs.version }}.zip"
-          cd build
-          zip -r "../${ARCHIVE}" .
+          cd docs/build
+          zip -r "../../${ARCHIVE}" .
 
       - name: Upload docs artifact
         uses: actions/upload-artifact@v7

.github/workflows/docs.yml

@@ -62,7 +62,7 @@ jobs:
           path: |
             node_modules
             ~/.bun/install/cache
-          key: bun-docs-${{ runner.os }}-${{ hashFiles('bun.lock') }}
+          key: bun-docs-${{ runner.os }}-${{ hashFiles('docs/bun.lock') }}
           restore-keys: |
             bun-docs-${{ runner.os }}-
 
@@ -99,3 +99,4 @@ jobs:
       - name: Deploy to GitHub Pages
         id: deployment
         uses: actions/deploy-pages@v5
+s: actions/deploy-pages@v5