site: publish the platform guides at /platforms/ and link them from an About page

(cherry picked from commit 408d6cdfd76d13945db1d9727b58feca23d1e48b)

internal/site/platforms_test.go

@@ -0,0 +1,42 @@
+package site
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestRenderPlatformsShowsTheGuides(t *testing.T) {
+	root, err := filepath.Abs(filepath.Join("..", ".."))
+	if err != nil {
+		t.Fatal(err)
+	}
+	out := t.TempDir()
+	count, err := RenderPlatforms(root, out)
+	if err != nil {
+		t.Fatalf("RenderPlatforms: %v", err)
+	}
+	if count != 3 {
+		t.Fatalf("got %d guides, want 3", count)
+	}
+	raw, err := os.ReadFile(filepath.Join(out, "platforms", "index.html"))
+	if err != nil {
+		t.Fatalf("read page: %v", err)
+	}
+	page := strings.ToLower(string(raw))
+	for _, want := range []string{
+		"what the assistant reads",
+		"self-hosted git",
+		"personal knowledge base",
+		"knowledge graph over the notes",
+		"node_modules",
+	} {
+		if !strings.Contains(page, want) {
+			t.Errorf("page does not contain %q", want)
+		}
+	}
+	if _, err := os.Stat(filepath.Join(out, "platforms", "index.json")); err != nil {
+		t.Errorf("index.json missing: %v", err)
+	}
+}