docs: fix contribution link (#12189)

* web: Add InvalidationFlow to Radius Provider dialogues

## What

- Bugfix: adds the InvalidationFlow to the Radius Provider dialogues
  - Repairs: `{"invalidation_flow":["This field is required."]}` message, which was *not* propagated
    to the Notification.
- Nitpick: Pretties `?foo=${true}` expressions: `s/\?([^=]+)=\$\{true\}/\1/`

## Note

Yes, I know I'm going to have to do more magic when we harmonize the forms, and no, I didn't add the
Property Mappings to the wizard, and yes, I know I'm going to have pain with the *new* version of
the wizard. But this is a serious bug; you can't make Radius servers with *either* of the current
dialogues at the moment.

* docs: fix link from project root to the Contributing documentation in our product.

CONTRIBUTING.md

@@ -1 +1 @@
-website/developer-docs/index.md
+website/docs/developer-docs/index.md