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

(cherry picked from commit 408d6cdfd76d13945db1d9727b58feca23d1e48b)

internal/site/build.go

@@ -100,6 +100,13 @@ func (s *Site) Build() error {
 	if err := writePage("/404.html", templates.NotFoundPage(s.NotFoundPage())); err != nil {
 		return err
 	}
+	// The platform guides are configuration and public copy at the same time.
+	// Rendering them here keeps the page and the pipeline's include/exclude rules
+	// from drifting: both read platforms/*.yml.
+	if _, err := RenderPlatforms(s.Config.Root, s.Config.OutDir); err != nil {
+		return fmt.Errorf("platform guides: %w", err)
+	}
+	pages++
 
 	for _, f := range s.Fragments {
 		if err := writeFile(filepath.Join(s.Config.OutDir, filepath.FromSlash(f.File)), []byte(f.HTML)); err != nil {