ci: setup docs prose linting with vale

- only deploy if prose lint passes
- rework styles download
- add vale anotations matcher
- set MinAlertLevel to warning
- add website pages to the prose linter
- do not fail on docs linting

.github/annotations/vale.json

@@ -0,0 +1,18 @@
+{
+  "problemMatcher": [
+    {
+      "severity": "error",
+      "pattern": [
+        {
+          "regexp": "^([^:]+):(\\d+):(\\d+):([^:]+):(.*)$",
+          "file": 1,
+          "line": 2,
+          "column": 3,
+          "code": 4,
+          "message": 5
+        }
+      ],
+      "owner": "vale-error"
+    }
+  ]
+}