Fix syntax and meta information

Author: stevepiercy

docs/backend/index.md

@@ -1,9 +1,9 @@
 ---
 myst:
   html_meta:
-    "description": ""
-    "property=og:description": ""
-    "property=og:title": "Backend"
+    "description": "Plone 6 Backend"
+    "property=og:description": "Plone 6 Backend"
+    "property=og:title": "Plone 6 Backend"
     "keywords": "Plone 6, Volto, Classic UI, frontend, backend, plone.restapi, plone.api"
 ---
 

docs/backend/upgrading/add-on-upgrade.md

@@ -33,7 +33,7 @@ This chapter describes the steps to migrate your third party products.
 If `/prefs_install_products_form` is unreachable, and you are using Plone 5.0.x or earlier, you should try doing the add-on upgrades from the Management Interface.
 Navigate to the `portal_quickinstaller` in the Management Interface at `/portal_quickinstaller/manage_installProductsForm`, and reinstall or upgrade products that are shown to be outdated.
 
-```{versiondeprecated} 5.1
+```{deprecated} 5.1
 `portal_quickinstaller` was deprecated in Plone 5.1, and was removed in Plone 6.0.
 See [PLIP 1775](https://github.com/plone/Products.CMFPlone/issues/1775) and {ref}`upgrade-5.1-do-not-use-portal_quickinstaller-label` for Plone 5.1 and later.
 ```

docs/classic-ui/theming/through-the-web.md

@@ -4,7 +4,7 @@ myst:
     "description": "Through-the-web (TTW) theme customization in Plone 6 Classic UI"
     "property=og:description": "Through-the-web (TTW) theme customization in Plone 6 Classic UI"
     "property=og:title": "Through-the-web (TTW) theme customization in Plone 6 Classic UI"
-    "keywords": ""
+    "keywords": "Plone 6, Classic UI, Through-the-web, TTW, theme, customization"
 ---
 
 (classic-ui-through-the-web-label)=