Dependabot: Ignore Sphinx upgrades due to ReadTheDocs on v1 (#4486)

Co-authored-by: Hugo Locurcio <[email protected]>

.github/dependabot.yml

@@ -8,3 +8,6 @@ updates:
     directory: "/"
     schedule:
       interval: "daily"
+    ignore:
+      # ReadTheDocs is staying on v1.
+      - dependency-name: "sphinx"