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/filter.md

@@ -7,7 +7,7 @@ og_image:  https://docs.datacontroller.io/img/filter_dynamic_on.png
 
 # Filtering
 
-Data Controller for SAS® enables you to create complex table filters. The "dynamic" setting enables the dropdown values to be pre-filtered by previous filter clauses.  Filtered views are shareable!  They are also used when [embedded in VA.](/embed-va)
+Data Controller for SAS® enables you to create complex table filters. The "dynamic" setting enables the dropdown values to be pre-filtered by previous filter clauses.  Filtered views are shareable!  They are also used when [embedded in VA.](/visualanalytics/)
 
 The clause shown above the grid is the resolved WHERE clause that was applied, so it uses SAS syntax - a date, datetime or time filter reads as a SAS literal, eg `SOME_DATE = '20APR2042'd`, rather than the internal day or second count.