[Refactor] update CONTRIBUTION for new theme

CONTRIBUTING.md

@@ -20,11 +20,7 @@ Each page has the following frontmatter:
 - `categories`: (Optional) Meta-data about the topic
 - `aliases`: URIs from old pages. Configure redirects here.
 - `hidden`: If `true` page does not display in sidebar. Works only on leaf pages.
-- `og_image` (Optional) path to [og:image](https://ogp.me/). Default is Rhize logo). Default is Rhize logo
-- `menu.main`: An object that configures the sidebar main menu properties
-  - `name`:  (Optional) The title of the page as it appears in the sidebar. Default is `title`
-  - `identifier`: How to reference the page in menu configs, and relrefs and other shortcodes
-  - `parent`: The parent page (use its `identifier`)
+- `images` (Optional) Array of images. First is [og:image](https://ogp.me/). Default og:image is Rhize logo
 
 
 
@@ -36,11 +32,8 @@ categories: ["reference"]
 weight: 930
 aliases:
   - "/old/page/" # use trailing slash
-menu:
-  main:
-    name: GQL types and filters
-    identifier: gql-filters
-    parent: reference
+images:
+  - "/images/og/image.png"
 ```
 
 
@@ -79,6 +72,9 @@ Hugo processes their instructions to render something on the page.
 
 Common use cases are for style, as with notices, and for templating.
 
+Some shortcodes are [built-in to Hextra theme](https://imfing.github.io/hextra/docs/guide/shortcodes/)
+
+
 ### Abbreviation
 
 The abbreviation provides tooltips, using definitions given in `data/termbase.yaml`.