docs: separate open collective and sponsors

docs/open-collective.md

@@ -0,0 +1 @@
+<script src="https://opencollective.com/nix-community/banner.js"></script>

docs/sponsors.md

@@ -1,9 +1,5 @@
 Thanks to all our sponsors.
 
-<object type="image/svg+xml" data="https://opencollective.com/nix-community/tiers/backers.svg?avatarHeight=55&width=500"></object>
-
-<object type="image/svg+xml" data="https://opencollective.com/nix-community/tiers/sponsors.svg?avatarHeight=55&width=500"></object>
-
 <!-- prettier-ignore-start -->
 |[<img src="https://raw.githubusercontent.com/cachix/docs.cachix.org/master/source/logo.png" width="200" alt="Cachix">](https://cachix.org)|[<img src="https://raw.githubusercontent.com/Gandi/.github/b1f21a402d9223c672476b41148429f538be5303/logos/black.svg" width="200" alt="Gandi">](https://www.gandi.net/)|
 |:-:|:-:|

mkdocs.yml

@@ -1,5 +1,6 @@
 nav:
   - index.md
+  - Open Collective: open-collective.md
   - sponsors.md
   - Services:
       - continuous-integration.md