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

docs/index.md

@@ -42,7 +42,7 @@ Data Controller is regularly updated with new features.  If you see something th
 * [Row Level Security](/row-level-security)
 * Excel [formula support](excel)
 * Dynamic [cell dropdown](/dynamic-cell-dropdown)
-* [SAS Visual Analytics embed](/embed-va/) - drive the editor from VA report selections
+* [SAS Visual Analytics embed](/visualanalytics/) - drive the editor from VA report selections
 * Works on ALL flavours of SAS (Base, EBI, Viya)