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_plugin_test.go

@@ -0,0 +1,34 @@
+package party_test
+
+import (
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/pact"
+)
+
+// BlogPlugin is the acme.blog plugin: the Go form of a WinterCMS Plugin.php.
+// A real plugin package also registers it from init with
+// party.Register(&BlogPlugin{}).
+type BlogPlugin struct{}
+
+// The optional capabilities the plugin opts into, checked at compile time.
+var _ pact.HasPermissions = (*BlogPlugin)(nil)
+
+// ID is the plugin identifier in vendor.plugin form.
+func (p *BlogPlugin) ID() string { return "acme.blog" }
+
+// Requires lists the plugins that must register and boot first ($require).
+func (p *BlogPlugin) Requires() []string { return []string{"acme.user"} }
+
+// Register runs before any plugin boots: publish services here.
+func (p *BlogPlugin) Register(app *backpack.App) error { return nil }
+
+// Boot runs after every plugin registered: listen to events and look up
+// services other plugins published.
+func (p *BlogPlugin) Boot(app *backpack.App) error { return nil }
+
+// Permissions replaces registerPermissions().
+func (p *BlogPlugin) Permissions() []pact.Permission {
+	return []pact.Permission{
+		{Code: "acme.blog.access_posts", Tab: "Blog", Label: "Manage posts"},
+	}
+}