feat(11.1-03): add the Coming from WinterCMS concept map with a verified plugin example

- docs/setup/coming-from-wintercms.md maps WinterCMS concepts to checked
  pkg.Ident spans and lists what SummerCMS does not provide
- party BlogPlugin and ExamplePlugin, shown through src= fences
- TestDocsRequiredPages asserts required pages build as .html and .md
- index links the new page

modules/party/example_test.go

@@ -0,0 +1,13 @@
+package party_test
+
+import (
+	"fmt"
+
+	"git.golem15.com/golem15/summercms/modules/party"
+)
+
+func ExamplePlugin() {
+	var p party.Plugin = &BlogPlugin{}
+	fmt.Println(p.ID())
+	// Output: acme.blog
+}