docs(va): move the embed page to /visualanalytics

- rename docs/embed-va.md -> docs/visualanalytics.md, so the page is served
  at /visualanalytics/
- redirect the old /embed-va/ path to the new one
- point the three inbound links (index, filter, deploy-viya) at the new path
- fill in og_title and og_image so link previews carry the page title and a
  card image instead of the site defaults
- the object's author often has elevated permissions, not usually an admin

mkdocs.yml

@@ -73,7 +73,7 @@ nav:
     - Selectboxes: dcc-selectbox.md
     - Tables: dcc-tables.md
     - Validations: dcc-validations.md
-    - Visual Analytics: embed-va.md
+    - Visual Analytics: visualanalytics.md
 - Macros: macros.md
 - Installation:
     - System Requirements: dci-requirements.md
@@ -113,6 +113,7 @@ plugins:
       redirect_maps:
           'dci-evaluation.md': 'dci-deploysas9.md'
           'dcu-fileupload.md': 'files.md'
+          'embed-va.md': 'visualanalytics.md'
 
 # Repository
 repo_name: 'dc/docs.datacontroller.io'