fix: move ExternalPlugin overrides before loadQuartzConfig in quartz.ts docs

loadQuartzConfig() internally calls loadQuartzLayout() to build the
PageTypeDispatcher, which instantiates components. Plugin option overrides
must be registered before this happens. Updated quartz.ts and all doc
snippets that show the ExternalPlugin override pattern to place calls
before loadQuartzConfig(). Also updated OxHugo and Roam docs from stale
v4 plugins pattern to v5 override pattern.

docs/plugins/CustomOgImages.md

@@ -43,12 +43,13 @@ For the TS override approach (needed for custom `imageStructure`):
 import * as ExternalPlugin from "./.quartz/plugins"
 import { defaultImage } from "./quartz/plugins/emitters/ogImage"
 
+// Must be placed before loadQuartzConfig()
 ExternalPlugin.CustomOgImages({
   colorScheme: "lightMode",
   width: 1200,
   height: 630,
   excludeRoot: false,
-  imageStructure: defaultImage, // custom JSX component — requires TS
+  imageStructure: defaultImage,
 })
 ```